U.S. patent application number 12/402257 was filed with the patent office on 2009-10-22 for nanowires and nanoribbons as subwavelength optical waveguides and their use as components in photonic circuits and devices.
This patent application is currently assigned to THE REGENTS OF THE UNIVERSITY OF CALIFORNIA. Invention is credited to Rong Fan, Matthew Law, Donald J. Sirbuly, Andrea Tao, Peidong Yang.
Application Number | 20090263912 12/402257 |
Document ID | / |
Family ID | 41201445 |
Filed Date | 2009-10-22 |
United States Patent
Application |
20090263912 |
Kind Code |
A1 |
Yang; Peidong ; et
al. |
October 22, 2009 |
NANOWIRES AND NANORIBBONS AS SUBWAVELENGTH OPTICAL WAVEGUIDES AND
THEIR USE AS COMPONENTS IN PHOTONIC CIRCUITS AND DEVICES
Abstract
A microfluidic optical sensor utilizes at least one
subwavelength nanowire or nanoribbon waveguide coupled to a fluidic
structure having at least one nanofluidic channel through which one
or more molecular species are conveyed. In response to optical
pumping (e.g., a laser source) the waveguide optically interrogates
nearby molecular species retained within said fluidic structure to
detect chemical species in response to optical characterization of
small (on the order of sub-picoliter) volumes of solution.
Characterization is performed in response to evanescent wave
sensing. In one aspect, optical characterization is selected from
the group of optical characterizations consisting of absorbance,
fluorescence and surface enhanced Raman spectroscopy (SERS).
Inventors: |
Yang; Peidong; (El Cerrito,
CA) ; Sirbuly; Donald J.; (Mountain House, CA)
; Fan; Rong; (Pasadena, CA) ; Law; Matthew;
(Irvine, CA) ; Tao; Andrea; (Santa Barbara,
CA) |
Correspondence
Address: |
JOHN P. O'BANION;O'BANION & RITCHEY LLP
400 CAPITOL MALL SUITE 1550
SACRAMENTO
CA
95814
US
|
Assignee: |
THE REGENTS OF THE UNIVERSITY OF
CALIFORNIA
Oakland
CA
|
Family ID: |
41201445 |
Appl. No.: |
12/402257 |
Filed: |
March 11, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2007/078021 |
Sep 10, 2007 |
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12402257 |
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11559244 |
Nov 13, 2006 |
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PCT/US2007/078021 |
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PCT/US2005/017029 |
May 13, 2005 |
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11559244 |
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60844015 |
Sep 11, 2006 |
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60844015 |
Sep 11, 2006 |
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60571416 |
May 13, 2004 |
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60643612 |
Jan 12, 2005 |
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Current U.S.
Class: |
436/164 ;
422/82.11; 977/700; 977/762 |
Current CPC
Class: |
B82Y 20/00 20130101;
G02B 6/107 20130101; B01L 3/5027 20130101; G01N 21/7703 20130101;
B82Y 15/00 20130101; G01N 21/648 20130101; G01N 21/6489 20130101;
G01N 21/658 20130101; G01N 2021/0346 20130101; G01N 21/05
20130101 |
Class at
Publication: |
436/164 ;
422/82.11; 977/762; 977/700 |
International
Class: |
G01N 21/00 20060101
G01N021/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0003] This invention was made with Government support under
Contract No. DE-FG02-02ER-46021 awarded by the Department of
Energy, Grant No. DE-AC02-05CH11231 awarded by the Department of
Energy. The Government has certain rights in this invention.
Claims
1. A microfluidic optical sensor utilizing a nanowire or nanoribbon
waveguide, comprising: at least one fluidic structure configured
for conveying one or more molecular species; at least one optical
waveguide retained proximal said fluidic structure; said at least
one optical waveguide configured with a sub-optical diameter and
positioned to optically interrogate nearby molecular species
retained within said fluidic structure; means for optically pumping
said at least one optical waveguide; and means for detecting
chemical species within said fluidic structure in response to
optical characterization of sub-picoliter volumes of solution.
2. A microfluidic optical sensor as recited in claim 1, wherein
said optical waveguide is retained proximal said fluidic channel by
bridging one or more sensing channels within said fluidic
structure.
3. A microfluidic optical sensor as recited in claim 1, wherein
said optical waveguide is configured for guiding emissions to a
position within said fluidic structure.
4. A microfluidic optical sensor as recited in claim 1: wherein
said optical characterization is performed in response to
subwavelength evanescent field optical sensing; and wherein said
optical characterization is selected from the group of optical
characterizations consisting of absorbance, fluorescence and
surface enhanced Raman spectroscopy (SERS).
5. A microfluidic optical sensor as recited in claim 1, wherein
said fluidic structure comprises a fluidic channel through which an
analyte, containing one or more molecular species, is retained
and/or communicated.
6. A microfluidic optical sensor as recited in claim 1, wherein
said means for optically pumping comprising the optical output of a
laser source.
7. A microfluidic optical sensor as recited in claim 1, wherein
said means for detecting chemical species comprises evanescent wave
sensors operating in absorbance and fluorescence modes.
8. A microfluidic optical sensor as recited in claim 1, wherein
said means for detecting chemical species comprises an objective
through which optical energy is routed to a spectrometer.
9. A microfluidic optical sensor as recited in claim 1, wherein
said means for detecting chemical species is configured to detect
and identify one or more molecular species.
10. A microfluidic optical sensor as recited in claim 1, further
comprising metallic nanoparticles retained adjacent said optical
waveguide, the combination configured for exciting the metallic
nanoparticles in amplified Raman scattering.
11. A microfluidic optical sensor as recited in claim 1, further
comprising: a charged dye retained within an analyte retained by
said fluidic structure; and said charged dye configured to induce
fluorescence in said optical waveguide.
12. A microfluidic optical sensor as recited in claim 11, wherein
said charged dye is configured for electrostatic adherance to said
optical waveguide to attenuate optical waveguide fluorescence with
respect to time.
13. A microfluidic optical sensor as recited in claim 1, further
comprising means for pumping one or more molecular species through
said fluidic structure past said at least one optical
waveguide.
14. A microfluidic optical sensor as recited in claim 1, wherein
said fluidic structure comprises a flow cell structure having a
plurality of microfluidic channels.
15. A microfluidic optical sensor as recited in claim 1, wherein
said optical waveguide comprises a single crystalline nanoribbon
waveguide.
16. A microfluidic optical sensor as recited in claim 15: wherein
said fluidic structure comprises a flow cell structure having a
plurality of microfluidic channels; and wherein said optical
waveguide is coupled to said flow cell structure across said
microfluidic channels.
17. A microfluidic optical sensor as recited in claim 1: wherein
said optical waveguide comprises an SnO.sub.2 nanoribbon waveguide;
wherein said fluidic structure comprises a polydimethylsiloxane
(PDMS) microfluidic flow cell having a plurality of channels; and
wherein said SnO.sub.2 nanoribbon waveguide is positioned across
said channels attaching said waveguide to said flow cell.
18. A microfluidic optical sensor as recited in claim 1, wherein
said microfluidic optical sensor is configured for bonding to a
quartz substrate.
19. A microfluidic optical sensor as recited in claim 1, wherein
said at least one optical waveguide is configured with a diameter
less than the wavelength of light to be guided
20. A microfluidic optical sensor as recited in claim 1, wherein
said optical waveguide comprises a nanoribbon or nanowire having an
aspect ratio greater than approximately 1000.
21. A microfluidic optical sensor as recited in claim 1, wherein
said optical waveguide comprises a nanoribbon or nanowire having a
diameter ranging from approximately 100 nm to approximately 400
nm.
22. A microfluidic optical sensor as recited in claim 1, wherein
said optical waveguide comprises a nanoribbon or nanowire of a
material selected from the group of single-crystalline materials
consisting of SnO.sub.2, and ZnO.
23. A microfluidic optical sensor as recited in claim 1, wherein
said means for optical pumping directs a visible photoluminescence
(PL) emission into said optical waveguide, and is guidable through
said optical waveguide.
24. A microfluidic optical sensor as recited in claim 1, wherein
said optical waveguide comprises a nanoribbon or nanowire having a
substantially uniform rectangular cross-section.
25. A microfluidic optical sensor as recited in claim 1, wherein
said optical waveguide comprises a nanoribbon or nanowire having a
substantially rectangular cross section ranging from approximately
15 nm.times.5 nm to approximately 2 .mu.m.times.1 .mu.m.
26. A microfluidic optical sensor as recited in claim 1, wherein
said microfluidic optical sensor is configured for chip integration
to provide for on-chip chemical analysis or biological spectroscopy
in which small excitation and detection volumes are required.
27. A microfluidic optical sensor utilizing a nanowire or
nanoribbon waveguide, comprising: a fluidic structure configured
with at least one fluidic channel through which an analyte,
containing one or more molecular species, is retained or
communicated; at least one subwavelength optical waveguide retained
proximal said fluidic structure; wherein said optical waveguide
bridges one or more sensing channels within said microfluidic
structure and is configured for guiding emissions to a position
within said fluidic structure; said at least one optical waveguide
configured with a sub-optical diameter and positioned to optically
interrogate nearby molecular species retained within said fluidic
structure; means for optically pumping said at least one optical
waveguide; and means for detecting chemical species within said
fluidic structure in response to optical characterization of
sub-picoliter volumes of solution.
28. An optical sensing method, comprising: providing an optical
sensor comprising a flow cell structure having a plurality of
microfluidic channels and a nanowire or nanoribbon waveguide
positioned across said channels and coupled to said flow cell
structure; flowing a material through said microfluidic channels;
optically pumping the waveguide to generate evanescent wave
emission through said channels; and detecting one or more chemical
species in response to optically registering interaction between
said chemical species and said evanescent wave emission.
29. A method as recited in claim 28, wherein said nanoribbon or
nanowire waveguide has a diameter that is less than the wavelength
of light to be guided from said optically pumping.
30. A method as recited in claim 28, wherein said nanoribbon or
nanowire waveguide is configured to steer visible and ultraviolet
light through its subwavelength cavity.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from, and is a 35 U.S.C.
.sctn. 111(a) continuation of, co-pending PCT international
application serial number PCT/US2007/078021, filed on Sep. 10,
2007, incorporated herein by reference in its entirety, which
claims priority from U.S. provisional patent application Ser. No.
60/844,015 filed on Sep. 11, 2006, incorporated herein by reference
in its entirety. This application is also a continuation-in-part of
U.S. patent application Ser. No. 11/559,244 filed on Nov. 13, 2006,
incorporated herein by reference in its entirety, which claims
priority to U.S. provisional patent application Ser. No. 60/844,015
filed on Sep. 11, 2006. Priority is claimed to each of the
foregoing applications.
[0002] This application is related to PCT Publication No. WO
2008/033763, published on Mar. 20, 2008, incorporated herein by
reference in its entirety.
INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT
DISC
[0004] Not Applicable
NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION
[0005] A portion of the material in this patent document is subject
to copyright protection under the copyright laws of the United
States and of other countries. The owner of the copyright rights
has no objection to the facsimile reproduction by anyone of the
patent document or the patent disclosure, as it appears in the
United States Patent and Trademark Office publicly available file
or records, but otherwise reserves all copyright rights whatsoever.
The copyright owner does not hereby waive any of its rights to have
this patent document maintained in secrecy, including without
limitation its rights pursuant to 37 C.F.R. .sctn. 1.14.
BACKGROUND OF THE INVENTION
[0006] 1. Field of the Invention
[0007] This invention pertains generally to optical waveguides, and
more particularly to subwavelength evanescent field optical
sensors.
[0008] 2. Description of Related Art
[0009] Chemically synthesized nanowires represent a unique class of
building blocks for the construction of nanoscale electronic and
optoelectronic devices. Since nanowire synthesis and device
assembly are typically separate processes, nanowires permit more
flexibility in the heterogeneous integration of different materials
than standard silicon technology allows, although the assembly
itself remains a major challenge. The toolbox of nanowire device
elements is growing and currently includes various types of
transistors, light emitting diodes, lasers, and photodetectors.
While the electrical integration of simple nanowire circuits using
lithography has been demonstrated, optical integration, which
promises higher speeds and greater device versatility, remains
unexplored.
[0010] Photonics, the optical analogue of electronics, shares the
logic of miniaturization that drives research in semiconductor and
communications technology. The ability to manipulate pulses of
light within sub-micron spaces is vital for highly integrated
light-based devices, such as optical computers, to be realized.
Recent advances in using photonic bandgap and plasmonic phenomena
to control the flow of light show substantial promise in this
regard. However, both of these approaches typically rely on
difficult and costly lithographic processes for device fabrication
and are in their early stages of development.
[0011] Compact, reusable chemical sensors are highly desirable for
on-site detection in the field, including the identification of
water contaminants, hazardous biochemical compounds or blood-serum
content. Ideally, a sensing platform should be portable and employ
several complementary sensing modalities that allow quantitative
chemical identification of extremely small sample volumes. Optical
spectroscopy is a powerful analytical tool for characterizing
biological and chemical systems, but making a standard optical
laboratory portable is a major challenge. Recent advances have been
made in the synthesis and assembly of nanomaterials, wherein the
integration of these materials into functional device architectures
for sensing and monitoring can be considered. Of the well-studied
inorganic nanostructures, chemically synthesized one-dimensional
(1D) semiconductors have gained significant interest from the
photonics community as passive and active components for
miniaturized spectroscopic devices. This is due in part to their
ability to guide a significant portion of the confined
electromagnetic energy outside the cavity (i.e., in the evanescent
field) while operating below the diffraction limit of light. Since
the evanescent field travels efficiently through fluidic and air
dielectrics, it is possible to integrate the waveguides into
microfluidic devices and sense molecules located near the surface
of the cavity.
[0012] One-dimensional semiconductor nanomaterials offer unique
advantages over their zero- and two-dimensional counterparts
because their geometric shapes allow them to capture and guide
light over long distances. Trapping light in volumes smaller than
the wavelength of light is essential to the miniaturization of
optical characterization techniques. Materials currently being
studied for this purpose include photonic crystals, high-index
solids, and metal surfaces. However, engineering versatile,
reusable optical devices from materials such as photonic crystals
and metallic nanostructures remains challenging due to the
difficulty in performing spectroscopy with the guided optical
energy. In addition, the synthetic steps for producing these
materials tend to be labor-intensive and involve costly
lithographic techniques.
[0013] Fiber-based detection is a unique alternative to free-space
sensing because it localizes chemical recognition at the surface of
a waveguide. Among the most popular sensing schemes that rely on
the evanescent field of a fiber are absorption and fluorescence.
Typically these set-ups involve multimode silica fibers with
diameters much larger than the free-space wavelength of light. The
evanescent field in these experiments has been used to measure
refractive indices of liquids, monitor volatile compounds in water
and detect shifts in localized surface plasmon resonances of
coupled metal colloids. Recently, it has been proposed to use
subwavelength silica fibers in a Mach-Zehnder type interferometer
to detect index changes caused by molecules interacting with the
surface of the fibers. Though these various sensing configuration
are promising for high sensitivity, fast cycling times and
reversibility, they do not provide versatility in their
spectroscopic detection or enable a chemical read-out of the
analyte.
[0014] Accordingly, the importance of moving beyond fiber sensing
on/off detectors is vital toward developing materials that can
support multiple analytical modes for chemical identification. The
present invention fulfills that need as well as others and
overcomes shortcomings of prior solutions.
BRIEF SUMMARY OF THE INVENTION
[0015] The present invention is directed at assembling photonic
circuits from a collection of nanoribbon/nanowire elements that
assume different functions, such as light creation, routing and
detection. The subwavelength optical waveguides of the invention
are formed from a nanoribbon or nanowire having a diameter that is
less than the wavelength of light to be guided and are potentially
simpler while being versatile, such as serving as a fundamental
element of photonic circuits of various types.
[0016] Chemically synthesized nanoribbons and nanowires have
several features that make them good building blocks, including
inherent one-dimensionality, a variety of optical and electrical
properties, good size control, low surface roughness and, in
principle, the ability to operate both above and below the
diffraction limit. An important step toward integrated
nanoribbon/wire photonics is to develop a nanoribbon/wire waveguide
that can couple pairs of nanoribbon/wire elements and provide the
flexibility in interconnection patterns that is needed to carry out
complex tasks, such as logic operations.
[0017] The invention is amenable to being embodied in a number of
ways, including but not limited to the following
implementations.
[0018] At least one implementation is a microfluidic optical sensor
utilizing a nanowire or nanoribbon waveguide, comprising: (a) at
least one fluidic structure configured for conveying one or more
molecular species; (b) at least one optical waveguide retained
proximal said fluidic structure; (c) said at least one optical
waveguide configured with a sub-optical diameter and positioned to
optically interrogate nearby molecular species retained within said
fluidic structure; (d) means for optically pumping (e.g., laser
source) said at least one optical waveguide; and (e) means for
detecting chemical species within said fluidic structure in
response to optical characterization of sub-picoliter volumes of
solution.
[0019] A number of variations of the microfluidic optical sensor
can be implemented either separately or in combination, such as the
following. In one implementation said optical waveguide is retained
proximal said fluidic channel such as by bridging one or more
sensing channels within said fluidic structure. In at least one
implementation metallic nanoparticles are retained adjacent said
optical waveguide, the combination thus being configured for
exciting the metallic nanoparticles in amplified Raman scattering.
In at least one embodiment a charged dye is retained within the
analyte to alter fluorescence characteristics of the optical
sensor. For example, in one implementation the charged dye is
configured for electrostatic adherance to said optical waveguide to
attenuate optical waveguide fluorescence with respect to time. The
optical waveguide is configured for guiding emissions to a position
within said fluidic structure, with optical characterization
performed in response to subwavelength evanescent field optical
sensing. In one implementation, optical characterization is
selected from the group of optical characterization parameters
consisting of absorbance, fluorescence and surface enhanced Raman
spectroscopy (SERS). In one preferred implementation, the fluidic
structure comprises one or more fluidic channels through which an
analyte, containing one or more molecular species, is retained
and/or communicated. In at least one implementation the means for
detecting chemical species includes an optical objective through
which optical energy is routed to a spectrometer. In at least one
implementation a pumping means is provided for pumping one or more
molecular species through said fluidic structure past said at least
one optical waveguide.
[0020] At least one implementation is an optical sensing method,
comprising: (a) providing an optical sensor comprising a flow cell
structure having a plurality of microfluidic channels and a
nanowire or nanoribbon waveguide positioned across said channels
and coupled to said flow cell structure; (b) flowing a material
through said microfluidic channels; (c) optically pumping the
waveguide to generate evanescent wave emission through said
channels; and (d) detecting one or more chemical species in
response to optically registering interaction between said chemical
species and said evanescent wave emission. A number of variations
of the above method can also be implemented without departing from
the teachings of the present invention.
[0021] The present invention can provide a number of beneficial
aspects which can be implemented either separately or in any
desired combination without departing from the present
teachings.
[0022] An aspect of the invention describes the assembly of
photonic circuit elements from nanostructures such as SnO.sub.2
nanoribbon and ZnO nanowire waveguides.
[0023] Another aspect of the invention comprises high aspect ratio
(e.g., >1000) nanoribbons/nanowires waveguides having diameters
below the wavelength of light (typically 100 nm to 400 nm) for
internally generated photoluminescence (PL) and nonresonant
UV/visible light emitted from evanescently coupled structures.
[0024] Another aspect of the invention describes the length,
flexibility and strength of these single-crystalline structures
enable them to be manipulated and positioned on surfaces to create
various single-ribbon shapes and multi-ribbon optical networks,
including ring-shaped directional couplers and nanoribbon/wire
emitter-waveguide-detector junctions.
[0025] Another aspect of the invention is an ability to manipulate
the shape of active and passive nanoribbon/wire cavities to provide
a new tool for investigating the cavity dynamics of subwavelength
structures.
[0026] Another aspect of the invention pushes subwavelength optical
fibers beyond the use of silica nanoribbons/nanowires.
[0027] Another aspect of the invention teaches fabrication of
nanoribbon and nanowires from active, passive, non-linear and
semiconducting inorganic crystals, as well as a wide variety of
polymers, and combinations thereof.
[0028] Another aspect of the invention takes advantage of
simultaneous photon, charge carrier and spin manipulation within
and between nanowires of different compositions.
[0029] Another aspect of the invention takes advantage materials
having higher refractive indices than silica-based glasses, therein
permitting confinement of light of a given wavelength within
thinner structures for denser integration.
[0030] Another aspect of the invention is waveguiding in liquids
using subwavelength nanoribbon/wire optical waveguides.
[0031] Another aspect of the invention provides for the
manipulation of freestanding nanoribbons/wires as mechanically
flexible elements on surfaces or as mobile probes in fluids
providing versatility beyond that of lithographically-defined
structures affixed to substrates.
[0032] Another aspect of the invention is a nanoribbon/wire optical
waveguide having a high aspect ratio and a diameter less than the
wavelength of light to be guided, for example an aspect ratio
greater than approximately 1000, with a diameter in the range from
approximately 100-400 nm.
[0033] Another aspect of the invention is a subwavelength optical
waveguide formed from a crystalline oxide nanoribbon/wire, for
example SnO.sub.2, ZnO, or GaN.
[0034] Another aspect of the invention is to provide a
nanoribbon/wire laser and a nanoribbon/wire photodetector coupled
by a nanoribbon/wire optical channel.
[0035] Another aspect of the invention is an optical waveguide
comprising a nanoribbon/wire dispersed on an SiO.sub.2 or mica
substrate.
[0036] Another aspect of the invention is a method of forming a
SnO.sub.2 nanoribbon/wire waveguide.
[0037] Another aspect of the invention is a method of forming a ZnO
nanoribbon/wire waveguide.
[0038] Another aspect of the invention is an apparatus for guiding
light through liquid media, for example through a nanoribbon or
nanowire waveguide comprising SnO.sub.2, ZnO, high dielectric
waveguides, GaN, and so forth.
[0039] Another aspect of the invention is a probe or a sensor
comprising a subwavelength nanostructure waveguide.
[0040] Another aspect of the invention is an optical router
comprising at least two coupled nanoribbon waveguides, such as
SnO.sub.2, or ZnO nanowire waveguides.
[0041] Another aspect of the invention is an optical router
comprising a network of nanoribbon waveguides (e.g., SnO.sub.2 and
so forth) configured to separate white light and route individual
colors based on a short-pass filtering effect.
[0042] Another aspect of the invention is an optical crossbar grid
comprising two pairs of orthogonal nanoribbon waveguides configured
to conduct light through abrupt 90.degree. angles, for example
SnO.sub.2 nanoribbon waveguides.
[0043] Another aspect of the invention are on-chip photonic
sensors, with structures smaller than the wavelength of the guided
light, which control the flow of light through liquids.
[0044] Another aspect of the invention are semiconductor nanowire
waveguides which provide excellent optical confinement in solutions
and can be integrated into microfluidic devices.
[0045] Another aspect of the invention is a microfluidic optical
sensor that employs a nanowire or nanoribbon waveguide.
[0046] Another aspect of the invention is an optical sensor having
a plurality of microfluidic channels proximally coupled to a
nanowire or nanoribbon waveguide.
[0047] Another aspect of the invention is an optical sensor formed
from a flow cell structure having a plurality of microfluidic
channels, and a single crystalline nanoribbon waveguide coupled to
the structure across the microfluidic channels.
[0048] Another aspect of the invention is a method of fabricating a
microfluidic sensor.
[0049] Another aspect of the invention is a sensor having flow cell
structure having a plurality of microfluidic channels, providing a
nanowire or nanoribbon waveguide, positioning the waveguide across
the channels, and coupling the waveguide to the flow cell
structure.
[0050] Another aspect of the invention is a sensor fabricated
having a nanoribbon waveguide, such as SnO.sub.2, with a
microfluidic flow cell, such as polydimethylsiloxane (PDMS), having
a plurality of channels upon which the nanoribbon waveguide is
attached.
[0051] Another aspect of the invention is an optical sensing method
based on evanescent wave emission.
[0052] Still another aspect of the invention is an optical sensing
method in which evanescent wave emissions are generated from an
optically pumped nanowire or nanoribbon waveguide positioned across
the channels of a flow cell through which material flows.
[0053] Further aspects of the invention will be brought out in the
following portions of the specification, wherein the detailed
description is for the purpose of fully disclosing preferred
embodiments of the invention without placing limitations
thereon.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0054] The invention will be more fully understood by reference to
the following drawings which are for illustrative purposes
only:
[0055] FIG. 1A-1B are images illustrating optical waveguiding in a
715 .mu.m long SnO.sub.2 nanoribbon.
[0056] FIG. 1C is a graph of emission spectra for the optical
waveguide of FIG. 1A-B.
[0057] FIG. 2A-2F are images illustrating panchromatic waveguiding
in a 425 .mu.m long nanoribbon.
[0058] FIG. 3A-3F are images illustrating shape manipulation of
nanoribbon waveguides.
[0059] FIG. 3G is a graph of PL spectra for the nanoribbon of FIG.
3A-F bent into different shapes.
[0060] FIG. 4A-4G are images illustrating a nanoribbon suspended
above a substrate undergoing physical manipulation by an etched
tungsten probe according to an aspect of the present invention.
[0061] FIG. 4H is a graph of intensity response to bending of the
nanoribbon shown in FIG. 4A-G.
[0062] FIG. 5A-5F are images illustrating dark-field images taken
before and after nanoribbon cavity shape manipulation according to
an aspect of the present invention.
[0063] FIGS. 6A, 6C are images illustrating nanoribbon coupling,
optical components and devices according to an aspect of the
present invention.
[0064] FIG. 6B is a graph of raw emission spectra of the left
nanoribbon before and after forming the junction as depicted in
FIG. 6A.
[0065] FIG. 7A-7B show optical coupling between a ZnO nanowire and
a SnO.sub.2 nanoribbon waveguide according to an aspect of the
present invention.
[0066] FIG. 7C is a graph of emission spectra of the coupled
structure as depicted in FIG. 7A-7B.
[0067] FIG. 8A is an image illustrating a hetero-junction created
between a single ZnO nanowire and a SnO.sub.2 nanoribbon according
to an aspect of the present invention.
[0068] FIG. 8B is a graph of spectra collected while pumping either
nanowire depicted in FIG. 8A.
[0069] FIG. 9A-9C are images illustrating a SnO.sub.2/SnO.sub.2
junction created by coupling two nanoribbon waveguides at their end
facets according to an aspect of the present invention.
[0070] FIG. 10A-10B are graphs of frequency response of nanoribbon
short-pass filters according to an aspect of the present invention
according to an aspect of the present invention.
[0071] FIG. 11A-11C are images illustrating waveguiding in water
according to an aspect of the present invention according to an
aspect of the present invention.
[0072] FIG. 12A-12B are images illustrating dark field images of
waveguiding in water according to an aspect of the present
invention.
[0073] FIGS. 13A, 13C are images illustrating fluorescence and
absorbance detection of R6G with a nanoribbon cavity according to
an aspect of the present invention.
[0074] FIGS. 13B, 13D are graphs of droplet region spectra for the
nanoribbon cavities shown in FIGS. 13A, 13C.
[0075] FIG. 14A is a schematic of SERS sensing with subwavelength
waveguides according to an aspect of the present invention.
[0076] FIG. 14B-14C are images illustrating a nanoribbon coated
with nanoparticles, such as represented in FIG. 14A, according to
an aspect of the present invention.
[0077] FIG. 15 is an image illustrating PL/dark-fields of two
nanoribbons evanescently coupled according to an aspect of the
present invention.
[0078] FIG. 16A is a schematic illustrating an example of
integrating waveguides into a fluidic device according to an aspect
of the present invention.
[0079] FIG. 16B-16C are images illustrating portions of the device
represented in FIG. 16A.
[0080] FIG. 17A-17E are images illustrating the routing of GaN PL
and lasing emission according to an aspect of the present
invention.
[0081] FIG. 17F is a graph of spectral comparison of lasing
emission before and after passage through the cavity as represented
in FIG. 17A-17F.
[0082] FIG. 18A are images illustrating multi-laser waveguiding
according to an aspect of the present invention.
[0083] FIG. 18B is a graph of guided light spectrum for the
waveguide of FIG. 18A.
[0084] FIG. 19A is a graph of emission spectra at different pump
fluence according to an aspect of the present invention.
[0085] FIG. 19B is a graph of energy curve for the same nanowire as
for FIG. 19A, and shown with an inset image of lasing emission from
a different nanowire, according to an aspect of the present
invention.
[0086] FIG. 20A-20E are images illustrating color filtering in a
nanoribbon network according to an aspect of the present
invention.
[0087] FIG. 21 is a graph depicting typical PL spectrum of a
SnO.sub.2 nanoribbon, showing its two defect bands according to an
aspect of the present invention.
[0088] FIG. 22A-22B are images illustrating optical routing in a
rectangular nanoribbon grid according to an aspect of the present
invention.
[0089] FIG. 23A-23B are images illustrating a device layout of an
evanescent field sensor according to the present invention
according to an aspect of the present invention.
[0090] FIG. 23C is a schematic of the device of FIG. 23A-23B.
[0091] FIG. 24A is a schematic of an absorbance geometry
exemplified as nanoribbons proximal to an evanescent field and
molecules, according to an aspect of the present invention.
[0092] FIG. 24B-24E are graphs of characteristic for devices
fabricated according to FIG. 24A.
[0093] FIG. 24F-24H are images of photoluminescence images
illustrating for molecules flowing past a sensor exemplified by
FIG. 24A.
[0094] FIG. 25A-25C are images illustrating dielectric scattering
and refractive index sensing with silver nanoparticles according to
an aspect of the present invention.
[0095] FIG. 25D is a graph of raw scattering spectra for the images
shown in FIG. 25A-C.
[0096] FIG. 26A is a schematic and associated Raman shift graph for
a nanoribbon evanescent wave SERS sensor according to an aspect of
the present invention.
[0097] FIG. 26B are graphs of Raman shift and associated images of
resonant SERS spectra according to the sensor configuration of FIG.
26A.
[0098] FIG. 26C is a graph of SERS spectra according to the sensor
configuration of FIG. 26A.
[0099] FIG. 27A-27B are graphs of absorbance spectra of a
positively charged dye (e.g., rhodamine 6G) according to an aspect
of the present invention.
[0100] FIG. 28A is a schematic of multiple coupled nanoribbons
configured for multi-pass absorption according to an aspect of the
present invention.
[0101] FIG. 28B-28C are images illustrating two coupled nanoribbons
for multi-pass absorption as represented in FIG. 28A.
[0102] FIG. 28D is a graph of PL intensity for the coupled
nanoribbons of FIG. 28A.
DETAILED DESCRIPTION OF THE INVENTION
1. Subwavelength Optical Waveguides
[0103] Nanoscale ribbon-shaped crystals of binary oxides exhibit a
range of interesting properties including extreme mechanical
flexibility, surface-mediated electrical conductivity, and lasing.
However, as part of a recent study of the photoluminescence (PL) of
SnO.sub.2 nanoribbons in our laboratory, we discovered that
nanoribbons with high aspect ratios (>1000) act as excellent
waveguides of their visible PL emission. SnO.sub.2 is a
wide-bandgap (3.6 eV) semiconductor characterized by PL bands at
2.5 eV (green) and 2.1 eV (orange), and finds application in gas
sensors and transparent electrodes. For our studies, we used
conventional thermal transport techniques to synthesize
single-crystalline nanoribbons of SnO.sub.2 with lengths of up to
5000 .mu.m. The structures synthesized possessed fairly uniform
(+/-10%) rectangular cross-sections with dimensions as large as 2
.mu.m.times.1 .mu.m and as small as 15 nm.times.5 nm. Many of the
nanoribbons synthesized were 100 nm to 400 nm wide and thick, which
we found to be an optimal size range for efficient steering of
visible and ultraviolet light in a subwavelength cavity.
[0104] Additionally, we have found that photonic circuit elements
can be assembled from, for example, SnO.sub.2 nanoribbon and ZnO
nanowire waveguides. High aspect ratio nanoribbons/wires with
diameters below the wavelength of light (typically 100 nm to 400
nm) were found not only to act as excellent waveguides of both
their own internally generated photoluminescence (PL), but also
nonresonant UV/visible light emitted from adjacent, evanescently
coupled, nanowires or external laser diodes. Furthermore, the
length, flexibility and strength of these single-crystalline
structures enable them to be manipulated and positioned on surfaces
to create various single-ribbon shapes and multi-ribbon optical
networks, including ring-shaped directional couplers and nanowire
emitter-waveguide-detector junctions. This ability to manipulate
the shape of active and passive nanowire cavities provides a new
tool for investigating the cavity dynamics of subwavelength
structures. Moreover, future advances in assembling the diverse set
of existing nanowire building blocks could lead to a novel and
versatile photonic circuitry.
[0105] It should be appreciated that the use nanoribbons/wires as
optical waveguides is based on the nanoribbons/wires having
diameters which are smaller than the wavelength of light. It should
also be noted that nanoribbons/wires need not have circular
cross-sections for use herein. For example, ZnO nanowires typically
have a hexagonal cross-section and SnO.sub.2 nanoribbons typically
have a rectangular cross-section. Therefore, in the case of a
non-circular cross-section, the term "diameter" is intended
generally to refer to the effective diameter, as defined by the
average of the major and minor axis of the cross-section of the
structure. However, the term "diameter" is not limited to the
foregoing definition and is also intended to encompass dimensions
of a nanoribbon/wire which allow for the nanoribbon/wire to
function as a subwavelength waveguide.
2. Nanoribbon Waveguides
[0106] Initially, waveguiding behavior of individual nanoribbons
dispersed on SiO.sub.2 and mica substrates were studied using
far-field microscopy and spectroscopy.
[0107] FIG. 1A-1C and FIG. 2A-2F illustrate representative data
collected from single nanoribbons with lengths of 715 and 425
.mu.m, respectively.
[0108] More particularly, FIG. 1A-1C illustrate optical waveguiding
in a 715 .mu.m long SnO.sub.2 nanoribbon that we synthesized. FIG.
1A is a dark-field image of a (350 nm wide by 245 nm thick)
meandering nanoribbon 10 and its surroundings, with a scale bar of
50 .mu.m. FIG. 1B is the PL image of the nanoribbon under laser
excitation. Here, the laser was focused to a spot size of
approximately 50 .mu.m at a 30.degree. incidence angle at the top
end of the nanoribbon. FIG. 1C shows the spectra of the emission
from the bottom terminus of the waveguide, collected at room
temperature and at a temperature of 5 K. A higher resolution
emission profile (inset) shows fine structure in three of the
central peaks. This fine structure was found to be present in every
peak.
[0109] FIG. 2 illustrates panchromatic waveguiding in a 425 .mu.m
long nanoribbon. FIG. 2A is a dark-field image of the nanoribbon
12, which has cross-sectional dimensions of 520 nm.times.275 nm,
with a scale bar is 50 .mu.m. FIG. 2B is a PL image with the UV
excitation spot centered near the middle of the nanoribbon, showing
waveguided emission from both ends. FIG. 2C is a magnified
dark-field PL view of the right end of the nanoribbon, with the
laser focused on the left end. A wide (.about.1 .mu.m) nanoribbon
14 lies across the nanoribbon of interest. The inset in FIG. 2C is
a scanning electron micrograph of the right terminus of the
nanoribbon, showing its rectangular cross-section. The scale bar is
500 nm. FIG. 2D, FIG. 2E and FIG. 2F are digital images of the
guided emission 16a, 16b, 16c, respectively, at the output end of
the nanoribbon during nonresonant excitation of the input end of
the nanoribbon with monochromatic light of wavelengths 652 nm
(red), 532 nm (green) and 442 nm (blue) light, respectively. The
leftmost emission spots 18a, 18b, 18c in FIG. 2D, FIG. 2E and FIG.
2F, respectively, were caused by scattering at the
nanoribbon-nanoribbon junction and were quenched by selectively
removing the wide nanoribbon 14 with a micromanipulator.
[0110] As can be seen, when in response to a tightly focused
continuous wave laser light (3.8 eV) onto one end of a nanoribbon,
a large fraction of the resulting PL was guided by the nanoribbon
cavity to its opposite end, where the PL emanated with high
intensity. Quite surprisingly, it was found that the nanoribbon
mimicked had many similarities. It was also found that nanoribbons
that were damaged internally during dispersion or which possessed
sizeable 3D surface defects scattered guided light in a series of
bright spots along their lengths. Referring to FIG. 2C, contact
points between nanoribbons were often dark, although overlying
nanoribbons, if thick, sometimes acted as scattering centers.
[0111] Referring again to FIG. 1C, it was also found that emission
spectrum collected from the end of a nanoribbon while exciting its
opposite end often featured complex, quasi-periodic modulation.
This is due to the transverse modes allowed in a planar waveguide
resulting from interference of electromagnetic waves resonating
within the rectangular cavity (i.e., an optical mode structure).
This modulation typically was found to be confined to the green PL
component in cases of simultaneous green and orange emission, which
suggests a difference in either the spatial location of PL emission
(i.e., bulk vs. surface) or confinement of the two colors in the
nanoribbon cavities. In short nanowire waveguides, such modulation
is due to longitudinal Fabry-Perot type modes, with a mode spacing
.DELTA..lamda. given by
.DELTA..lamda.=.lamda..sup.2/{2L[n-.lamda.(dn/d.lamda.)]}, where
.lamda. is the wavelength, L is the cavity length, and n is the
index of refraction (2.1 for SnO.sub.2). The nanoribbons, however,
were so long that .DELTA..lamda. was below the 0.01 nm resolution
limit of our instrumentation. In addition, SnO.sub.2 cavities are
unlikely to show longitudinal modes since the reflectivity of their
end facets is low (.ltoreq.13%) and there is no gain to compensate
for scattering and output-coupling losses. A systematic study of
the spectral structure is complicated by the complex dependence of
the modes on nanoribbon cross-sectional size and orientation
(through bend losses, substrate coupling and variations in
refractive index), as well as on light intensity and end facet
roughness. It should be noted that the existence of a mode
structure indicates that nanoribbon cavities are complex and
subject to a high level of finesse. In addition, as discussed
below, the loss at given wavelengths can be modified by distorting
the cavity shape.
[0112] In general, one would expect a subwavelength resonator to
show a large optical loss that is highly wavelength dependent, with
better confinement of shorter wavelength radiation. To investigate
the dependence of optical confinement on wavelength, single
nanoribbons were illuminated, such as with monochromatic red, green
and blue light at a 30.degree. incidence angle, and their end
emission was monitored. It was found that red waveguiding was rare,
green waveguiding was common, and blue waveguiding was ubiquitous.
We also found that, for a given dielectric material, cavity
geometry and wavelength, a critical diameter exists below which all
higher order optical modes are cut off and waveguiding becomes
increasingly difficult to sustain. More specifically, by treating a
nanoribbon waveguide as a cylinder of SnO.sub.2 embedded in air,
cutoff diameters were found for higher order transverse modes of
light at about 270 nm, 220 nm and 180 nm for the 652 nm, 532 nm and
442 nm. While this approximation simplifies the cavity shape and
ignores substrate coupling and other effects, these values are in
reasonable agreement with scanning electron microscopy measurements
of the sizes of the blue and green waveguides. The majority of
nanoribbons in our samples were too thin for propagating red light
over distances greater than approximately 100 .mu.m. However, it
was clearly found that nanoribbons with sufficiently large
cross-sectional dimensions as described above would effectively
guide wavelengths across the visible spectrum, acting as
subwavelength red-green-blue (RGB) optical fibers (e.g., optical
transmitters) as shown in FIG. 2D through FIG. 2F.
3. Wavelength-Dependent Loss
[0113] The wavelength-dependent loss of straight nanoribbons was
quantified using near-field scanning optical microscopy (NSOM). By
way of example and not limitation, nanoribbons were pumped (3.8 eV)
at different points along their length relative to a fixed
collection probe. We found that losses ranged from 1-8 dB mm.sup.-1
for wavelengths between 450 nm and 550 nm, depending on nanoribbon
cross-sectional area and the density of surface scattering centers.
These values are higher than those reported recently for
subwavelength silica waveguides, likely due to the somewhat rougher
nanoribbon surfaces and the extra loss due to substrate coupling.
It was noted, however, that the losses in this case are better than
what is required for integrated planar photonic applications, in
which waveguide elements are configured to transmit light over very
short distances.
4. Shape Manipulation
[0114] It has also been found that the nanoribbons used in for
these tests should be of sufficient length and strength to be
pushed, bent and shaped using a commercial micromanipulator under
an optical microscope. The large aspect ratio and elastic
flexibility of SnO.sub.2 nanoribbons allowed manipulating the
location and shape of individual nanoribbons under the optical
microscope using a commercial micromanipulator tipped with sharp
tungsten probes. Waveguiding nanoribbons with one free end
(dangling) in air could be elastically bent to large angles (e.g.,
up to about 180.degree.) without kinking or fracturing, which is
remarkable for an oxide that is brittle in its bulk form. In these
tests straight nanoribbons could be fashioned into wiggles, circles
and other shapes by using nanoribbon-substrate forces to prevent
elastic recoil.
[0115] The dragging, aligning and cutting of single nanoribbons is
routine. Here, the micromanipulator was used to selectively remove
the overlying nanoribbon in FIG. 2C and quench scattering from that
nanoribbon-nanoribbon interface. In a subsequent test the long
nanoribbon was diced into three equal segments, creating three
excellent waveguides.
[0116] FIG. 3A through FIG. 5F illustrate experimental results of
our shape manipulation of nanoribbon waveguides. If these
crystalline nanoribbon waveguides are to be useful as interconnects
in optical circuits, they should be configured for coupling light
from one nano-object to another and to be facilely transportable
from one location to another. To realize the latter, attempts were
made to bend and move the nanoribbons using the
micromanipulator.
[0117] FIG. 3A is an SEM image of a simple shape 20, demonstrating
the high level of positional control afforded by the
micromanipulator. This shape was created from a single straight
nanoribbon of dimensions 400 nm.times.115 nm that was cut into two
pieces and then assembled. FIG. 3B and FIG. 3C are optical images
of the emission end of a long nanoribbon (aspect ratio
approximately 5200), showing the minimal effect of curvature on
waveguiding. FIG. 3B is a black and white rendering of a true color
photograph taken after crafting a single bend. FIG. 3C is a
black-and-white dark-field/PL image captured after an S-turn was
completed. It was found that blue light could be guided around 1
.mu.m radii curves with low loss. The SEM image in the inset of
FIG. 3C resolves the bent geometry. FIG. 3D through FIG. 3F depicts
a series of dark-field images and FIG. 3G is the corresponding
guided PL spectra for a single nanoribbon 22 bent into different
shapes. Collection was at the right end of the nanoribbon in each
case. An unguided PL spectrum of the nanoribbon is included for
reference. Spectra are normalized and offset for clarity.
[0118] It will be appreciated that freestanding nanoribbons can be
repeatedly and elastically curved into loops with radii as small as
5 .mu.m, which is remarkable for a crystal that is brittle in its
bulk form. On appropriately chosen surfaces, single nanoribbons are
easily fashioned into a variety of shapes with the help of
nanoribbon-substrate forces to prevent elastic recoil as shown in
FIG. 3A. Careful manipulation is normally nondestructive to the
nanoribbon cavities. In practice, this manipulation method is
applicable to nanostructures that are free to move and visible
using dark-field microscopy, including, at the lower size limit,
short nanowires (e.g., 40 nm.times.3 .mu.m) and even large
nanocrystals. Though an inherently slow serial process, it is
faster and more versatile than similar approaches using, for
instance, scanning probes or in-situ scanning electron microscopy
manipulation. According to these aspects of the invention networks
of nanoribbon waveguides can be created and functioning
optoelectronic devices built by assembling individual nanowire
elements one at a time.
[0119] Manipulation also makes it possible to investigate the
shape-dependent waveguiding of single nanoribbon cavities. For
example, a tight S-turn was fashioned in one end of a long, thin
nanoribbon (dimensions: 785 .mu.m.times.275 nm.times.150 nm) to
illustrate the robust nature of optical steering in these
structures as shown in FIG. 3B and FIG. 3C. Losses around the bends
were small and did not noticeably reduce light output from the end
of the nanoribbon. In general, it was found that twists and bends
with radii of curvature as small as 1 .mu.m do not disrupt the
ability of these subwavelength waveguides to channel light across
hundreds of microns.
[0120] It was also observed that bending a nanoribbon, even
slightly, can dramatically change the mode structure of its output
light as shown in FIG. 3D through FIG. 3G. This most likely arises
as a result of a change in cavity curvature and/or cavity-substrate
coupling which alters the interference pattern of propagating
waves, resulting in the enhancement of some modes and the partial
quenching of others. The data also indicates that the emission
pattern from a typical nanoribbon is spatially heterogeneous, as
shown previously in ZnO nanowires. As a consequence, the far-field
spectrum changes somewhat with collection angle, though not enough
to account for the complex modal variations seen in response to
distortions of the cavity shape.
[0121] FIG. 4A-4H show an approximately 600 .mu.m long nanoribbon
24 slightly suspended above the substrate, which undergoes physical
manipulation by an etched tungsten probe. FIG. 4A, FIG. 4C, FIG. 4D
and FIG. 4F are dark-field images during the bending process, from
no bend (FIG. 4A) to a >90.degree. angle (FIG. 4F), illustrating
the extreme flexibility of the nanoribbons. FIG. 4B, FIG. 4E and
FIG. 4G are PL images taken at different bend angles. The
excitation source was focused on the top terminus of the nanoribbon
and light was guided through the bends to emerge at the bottom
terminus. FIG. 4H illustrates spectra taken at the bottom terminus
as a function of arbitrary bend angle. The curves identified as
Bend 1, 2, and 3 in FIG. 4H correspond to the images in FIG. 4C,
FIG. 4D and FIG. 4F, respectively. The mode structure was found to
be significantly dependent on the size and shape of the cavity.
[0122] The dark field images (FIG. 4A, FIG. 4C, FIG. 4D and FIG.
4F) were obtained during the process of bending a nanoribbon that
was slightly suspended above the substrate. This process was the
first direct indication of the degree of flexibility of these oxide
nanostructures. The corresponding PL images (FIG. 4B, FIG. 4E, and
FIG. 4G) provide additional information on the waveguiding behavior
of the cavity as the nanoribbon is bent to angles >90.degree..
In addition to the optical images, spectra were taken from the
waveguided terminus of the nanoribbon. FIG. 4H shows the resulting
emission profiles as a function of arbitrary bend angle. It is
apparent that the mode structure emerges as the semi-linear
nanoribbon begins to take physical shape, and leads to the
possibility of using these nanoribbons as high quality (Q) factor
cavities. To further pursue and explore the limitations of
physically perturbing these nanoribbons, thinner nanoribbons were
focused on that still exhibited outstanding waveguiding
properties.
[0123] FIG. 5A-5F clearly demonstrate the potential of these
structures in nano-photonic circuits. FIG. 5A and FIG. 5C are
dark-field images taken before (FIG. 5A) and after (FIG. 5C)
manipulating the cavity shape of a nanoribbon 26. The flexibility
of the nanoribbon allows it to maintain its shape integrity even
after the tungsten probe is removed. FIG. 5B and FIG. 5D are PL
images of the shapes in FIG. 5A and FIG. 5C, respectively. Even
with two sharp bends, the nanoribbon successfully guided the defect
emission from the left coupling end to the right terminus with
minimal loss occurring at the bend apexes. FIG. 5E and FIG. 5F are
dark-field/PL (FIG. 5E) and PL (FIG. 5F) images of a new nanoribbon
that had its bottom terminus pinned up against itself by the tip of
the manipulator. The excitation spot is just visible at the top of
the PL image and the bottom terminus is denoted by the bright spot
directly above the tungsten probe. Even under extreme curvatures of
radius, these nanoribbons were found to maintain their physical
structures and waveguiding properties.
[0124] The dark-field images (FIG. 5A and FIG. 5C) and
corresponding PL images (FIG. 5B and FIG. 5D) are before and after
illustrations of how these nanoribbons can be torqued into sharp
wiggles and curves, while still maintaining the low loss properties
of the originally shaped nanoribbon. FIG. 5E and FIG. 5F reveal
that this physical manipulation can be taken even further. Here,
the end terminus of a new nanoribbon is actually pinned up against
itself with the manipulator probe, leaving an exceptionally small
radius of curvature (<5 .mu.m) kink in the nanoribbon. Despite
the tight bend and physical contact with itself, the nanoribbon did
not exhibit any significant light loss due to scattering centers or
cavity leakage. By contrast conventional silica fibers are subject
to major problems with regard to scattering losses. When utilizing
a lower dielectric material, light confinement drastically breaks
down as critical angles are surpassed. In addition, any physical
contact with a material of like refractive index causes severe
energy loss. Tin oxide, however, can achieve a higher internal
confinement due to its higher index of refraction, nearly double
that of silica (2.3 to 1.4), and its unequivocal property of
minimizing loss at like-refractive index interfaces.
5. Nanoribbon Optical Couplers and Filters
[0125] FIG. 6A through FIG. 9C illustrate coupling of nanoribbon
waveguides for creating optical networks which may form the basis
of miniaturized photonic circuitry. The approximate size of a
nanoribbon can be inferred from the color of its guided PL; namely,
large nanoribbons are white, while small nanoribbons are blue. When
a nanoribbon of average size is pumped nearer to one end, it shines
blue at the far end and green at the near end, demonstrating the
higher radiation losses for longer wavelengths.
[0126] FIG. 10A-10B illustrates that this effect makes nanoribbons
excellent short-pass filters with tunable cutoffs based on path
length. Nanoribbon filters have been identified during testing
spanning the 465 nm to 580 nm region that feature steep cutoff
edges and virtually zero transmission of blocked wavelengths.
[0127] Since light diffracts in all directions when it emerges from
a subwavelength aperture, nanoribbons must be in close proximity,
and preferably in direct physical contact, to enable the efficient
transfer of light between them. Various coupling geometries were
tested and it was found that a staggered side-by-side arrangement,
in which two nanoribbons interact over a distance of several
micrometers, outperforms direct end-to-end coupling, which relies
on scattering between end facets. Staggered nanoribbons separated
by a thin air gap can communicate via tunneling of evanescent
waves. It is also possible to bond two nanoribbons together by Van
Deer Waals forces, for example by draping one over another, to
create a robust optical junction.
[0128] FIG. 6A-6C is illustrative of nanoribbon coupling for use in
optical components and devices. FIG. 6A depicts a black-and-white
dark-field/PL image of two coupled nanoribbons 28, 30 (both
nanoribbons are 750 nm.times.250 nm, 630 .mu.m total length). Light
is incident on the right terminus of the right nanoribbon 30 and
collected at the left terminus of the left nanoribbon 28. The arrow
denotes the location of the junction. The SEM image in the inset of
FIG. 6A resolves the junction layout. FIG. 6B illustrates raw
emission spectra of the left nanoribbon 28 before (upper curve) and
after (lower curve) forming the junction. The addition of the
second nanoribbon and the junction lowered the output light
intensity by only 50%, while its modulation was retained. FIG. 6C
is a black and white rendering of a true color PL image of a
three-ribbon ring structure that functions as a directional
coupler. The ring nanoribbon 32 (135 .mu.m.times.540 nm.times.175
nm) is flanked by two linear nanoribbons 34, 36 (34 at left, 120
.mu.m.times.540 nm.times.250 nm; 36 at right, 275 .mu.m.times.420
nm.times.235 nm). Light input at branch 1 exits preferentially at
branch 3 (as shown), while light input at branch 2 exits branch
4.
[0129] Note that FIG. 6A and FIG. 6B illustrate an example of
two-ribbon coupling. However, more functional geometries, such as
Y-junctions, branch networks, Mach-Zehnder interferometers and ring
oscillators can also be constructed. The three-ribbon ring
structure illustrated in FIG. 6C operates by circulating light that
is injected from one branch around a central cavity, which can be
tapped by one or more output channels to act as an optical hub.
With further integration, it should be possible to create optical
modulators based on nanoribbon assemblies that utilize the
electro-optic effect for phase shifting.
[0130] Single-crystalline nanoribbons are intriguing structures
with which to manipulate light, both for fundamental studies and
photonics applications. As passive elements, they are efficient
UV/visible waveguides and filters that can be assembled into
optical components, networks and devices. Being semiconductors or,
in their doped state, transparent metals, oxide nanoribbons are
well suited to combine simultaneous electron and photon transport
in active nanoscale components. Key challenges to the wider use of
these materials include narrowing their size dispersity and
developing better parallel assembly schemes for nanowire
integration. Answering the former challenge depends on gaining
control over the poorly understood vapor-solid process that is
typically used in nanoribbon synthesis.
[0131] FIG. 7A-7C illustrate successful optical coupling between a
ZnO nanowire 38 and a SnO.sub.2 nanoribbon waveguide 40. FIG. 7A is
a black and white rendering of a true color dark-field/PL image of
the nanowire 38 (56 .mu.m long, at top, pumped at 3.8 eV)
channeling light into the nanoribbon 40 (265 .mu.m long, at
bottom). The arrow denotes the location of the junction. FIG. 7B is
an SEM image of the nanowire/nanoribbon junction. FIG. 7C
illustrates spectra of the coupled structures taken at different
excitation and collection locations. From top to bottom: unguided
PL of the ZnO nanowire; waveguided emission from the ZnO nanowire
collected at the bottom terminus of the nanoribbon; waveguided
emission from the SnO.sub.2 nanoribbon excited just below the
junction and collected at its bottom terminus; unguided PL of the
SnO.sub.2 nanoribbon. Note that the emission from the ZnO nanowire
is modulated during its transit through the nanoribbon cavity.
[0132] FIG. 8A-8B illustrate another example of a hetero-junction
created between a single ZnO nanowire and a SnO.sub.2 nanoribbon.
FIG. 8A is a dark-field image of the junction after pushing a ZnO
nanowire up to the end facet of the SnO.sub.2 nanoribbon. The inset
in FIG. 8A is a magnification of the active coupling region showing
the short (.about.6-7 .mu.m) ZnO nanowire and the upper terminus of
the SnO.sub.2 nanoribbon. The total length of the nanoribbon was
approximately 600 .mu.m. FIG. 8B shows spectra collected at the
passive end (bottom terminus) while pumping either the ZnO nanowire
(On ZnO) or the SnO.sub.2 nanoribbon directly (On NR). A profile of
the band gap emission collected over the ZnO nanowire (ZnO Only) is
included for reference. The Modulation in the "On ZnO" spectrum is
a direct result of the broad emission from the ZnO propagating
through a high Q-factor SnO.sub.2 cavity.
[0133] The 50.times. dark-field image and 100.times. dark-field
inset of FIG. 8A pictorially demonstrate the basic components of an
active/passive nanophotonic device. However, to test that a
complete junction had been established between the two nanosystems,
the ZnO nanowire active end was optically pumped and collected at
the passive SnO.sub.2 nanoribbon end. As seen in FIG. 8B, ZnO band
gap emission created from the pump source was directed across the
intervening air space by the ZnO cavity and into the neighboring
SnO.sub.2 waveguide. The light output from the ZnO nanowire emerged
at the distant end of the nanoribbon and clearly showed a modulated
emission profile similar to the PL line shape seen in FIG. 4H. This
provides good evidence that the light was in fact waveguided across
hundreds of microns by the nanoribbon cavity. In building
like-material junctions, a similar manipulation scheme as described
above was employed. Two waveguiding nanoribbons were coupled with
their long axes collinear to each other by physically sliding a
larger nanoribbon directly adjacent to the far end of a smaller
nanoribbon.
[0134] FIG. 9 illustrates a SnO.sub.2/SnO.sub.2 junction created by
coupling two nanoribbon waveguides 42, 44 at their end facets. FIG.
9A and FIG. 9B are dark-field images before (FIG. 9A) and after
(FIG. 9B) completing a junction between a large 42 (.about.1 .mu.m)
and small 44 (.about.400 nm) diameter nanoribbon. FIG. 9C is a PL
image of the same nanoribbon junction and end terminus shown in
FIG. 9B demonstrating that multi-junction networks between
SnO.sub.2 nanoribbon waveguides can be realized.
[0135] The dark-field images in FIG. 9A and FIG. 9B capture the
junction before and after successfully adjoining the two
nanoribbons. The PL image in FIG. 9C verifies that light traveling
down the small nanoribbon can be directly coupled into a secondary
like-cavity. According to these technique we are now building
all-nanowire optical circuits that operate via electron injection
rather than optical pumping. The oxide waveguides serve as
important interconnects between active light sources, such as LEDs
and lasers, and optical detectors based on photoconducting
nanowires.
[0136] The optical loss of several nanoribbon waveguides was
measured by systematically varying the distance between UV
excitation (50 .mu.m spot size) and PL collection in the
near-field. A loss of about 2 dB mm.sup.-1 is estimated at a
wavelength of 550 nm for a nanoribbon with a 400.times.150 nm.sup.2
cross-section, which is significantly greater than losses reported
recently for subwavelength silica waveguides.
[0137] As can be seen from the forgoing, due to their extraordinary
length, high flexibility and strength, nanoribbon waveguides are
excellent materials with which to study the interplay between
mechanics, microstructure and optical confinement in nanoscale
cavities. They can be manipulated and assembled to serve as
photonic interconnects between single nano-objects, such as
nanowire lasers, in optical circuits and devices. Furthermore,
nanoribbon waveguides can be used as filter devices.
[0138] FIG. 10A-10B illustrate the use of nanoribbons as short-pass
filters. FIG. 10A shows room temperature PL spectra of five
different nanoribbons, each 200 .mu.m to 400 .mu.m long, with 50%
intensity cut-off wavelengths ranging from 465 nm to 580 nm.
Cross-sectional dimensions of the 465 nm, 492 nm, 514 nm, 527 nm
and 580 nm filters were 310 nm.times.100 nm (0.031 .mu.m.sup.2),
280 nm.times.120 nm (0.037 .mu.m.sup.2), 350 nm.times.115 nm (0.04
.mu.m.sup.2), 250 nm.times.225 nm (0.056 m.sup.2), and 375
nm.times.140 nm (0.053 .mu.m.sup.2), respectively. The spectra were
normalized and offset for clarity. FIG. 10B shows a series of
normalized emission spectra from a single nanoribbon (315
.mu.m.times.355 nm.times.110 nm) as the pump spot was scanned away
from the collection area. The unguided PL curve was obtained at a
pump-probe separation of 50 .mu.m. Larger separations resulted in a
progressive loss of the long wavelengths.
5.1 Example 1
[0139] SnO.sub.2 nanoribbon waveguides were synthesized by the
chemical vapor transport of SnO powder in a quartz tube reactor
operating at 1100.degree. C. and 350 Torr of flowing argon (50
sccm). Milligram quantities of nanoribbons were collected on an
alumina boat near the center of the reactor and deposited onto
clean substrates by dry transfer. Long ZnO nanowires were grown via
oxidation of Zn metal in a quartz furnace at 800.degree. C. and 760
Torr of flowing oxygen/argon, as described in the literature, and
also dispersed by dry transfer. InP nanowires produced by a
laser-assisted vapor-liquid-solid process (using Au catalyst) were
sonicated into ethanol solution and transferred to the surface by
drop-casting. Contacts to InP were fabricated by electron beam
lithography and thermal evaporation (100 nm Ti), followed by rapid
thermal annealing at 475.degree. C. in N.sub.2/H.sub.2 for one
minute.
5.2 Example 2
[0140] Optical measurements were carried out using a dark-field
microscope outfitted with a cryostat (Janis X-100). The PL
excitation source was a HeCd laser operating at 325 nm. Laser
pointers (532 nm and 652 nm) and the HeCd laser (442 nm) provided
nonresonant illumination. The size of the laser spot was
approximately 50 .mu.m for all measurements. Spectra were collected
with a fiber-coupled spectrometer (SpectraPro 300i, Roper
Scientific) and liquid N.sub.2 cooled CCD detector. Images were
captured using both a microscope-mounted camera (CoolSNAP, Roper
Scientific) and a handheld digital camera (PRD-T20, Toshiba). Loss
measurements were made with a commercial NSOM setup operating in
collection mode, with 325 nm excitation. For nanoribbon
manipulation, we used a three-axis commercial unit tipped with
tungsten probes (10 .mu.m ends).
[0141] As described above, photonic circuit elements can be
assembled from SnO.sub.2 nanoribbon and ZnO nanowire waveguides.
High aspect ratio nanoribbons/wires with diameters below the
wavelength of light (typically 100-400 nm) were shown to act as
excellent waveguides of both their own internally generated
photoluminescence (PL) and nonresonant UV/visible light emitted
from adjacent, evanescently coupled, nanowires or external laser
diodes. The length, flexibility and strength of these
single-crystalline structures enabled them to be manipulated and
positioned on surfaces to create various single-ribbon shapes and
multi-ribbon optical networks, including ring-shaped directional
couplers and nanowire emitter-waveguide-detector junctions. This
ability to manipulate the shape of active and passive nanowire
cavities provides a new tool for investigating the cavity dynamics
of subwavelength structures. Moreover, future advances in
assembling the diverse set of existing nanowire building blocks
could lead to a novel and versatile photonic circuitry.
6. Waveguiding in Liquids
[0142] In these tests it has been found, quite surprisingly, that
one-dimensional (1D) nanostructures can guide light through liquid
media. The fact that light can be delivered through these cavities
in solution offers a unique application for high dielectric
(n.ltoreq.2) waveguides in fluidic sensing and probing. Waveguiding
in liquids is especially important for integrated on-chip chemical
analysis and biological spectroscopy in which small excitation and
detection volumes are required. Subwavelength nanostructures can be
assembled to probe molecules in a fluorescence or absorption
scheme, both of which utilize the decaying light field outside of
the cavity to induce photon absorption. The waveguide is strongly
coupled to emitted photons near the cavity, allowing the generated
fluorescence to be directed back to the point of injection. Also,
the nanoscale dimensions of the waveguides afford small liquid
volumes (on the order of picoliters) to be sensed and presage the
way for miniaturized optical spectrometers.
[0143] The following discusses building upon the demonstrations of
nanowire/ribbon photonic assembly with several illustrative
implementations of optical routing between coupled nanowires. First
the ability to deliver individual nanosecond light pulses from
lasing GaN and ZnO nanowires through a nanoribbon waveguide is
shown. It should be appreciated that pulsed light must be
transmissible if nanowire photonic devices are to be useful in
communications or computing. Simple networks of SnO.sub.2
nanoribbons are then used to separate white light and route
individual colors based on a short-pass filtering effect. An
optical crossbar grid is also described which comprises two pairs
of orthogonal nanoribbons which conduct light through abrupt
90.degree. angles and provides a dramatic example of the nature of
optical confinement in these subwavelength cavities. The fact that
the waveguiding ability of these freestanding, flexible nanowires
and nanoribbons survives in liquid media suggests a role for
nanowire light delivery in microfluidics and biological
applications.
7. Subwavelength Waveguides as Optical Probes and Sensors
[0144] High dielectric subwavelength waveguides have a considerable
advantage for confining light in liquids over low dielectric
waveguides such as silica-based structures. The low index contrast
between the solution (cladding) and silica core
(n.sub.silica.apprxeq.1.45) hinders efficient propagation of the
light wave.
[0145] FIG. 11A-11C illustrate a comparison of the
photoluminescence (PL)/dark-field images of a SnO.sub.2 nanoribbon
(dimensions: 365 nm.times.105 nm.times.265 .mu.m) resting on a
silicon oxide surface (1 .mu.m thermal oxide) waveguiding in air
(n=1) and water (n=1.33). The PL is generated with a CW HeCd laser
(325 nm). These figures also illustrate how the guided PL spectrum
of this thin nanoribbon changes when it is immersed in water.
[0146] FIG. 11A is a combined PL/dark-field image of the nanoribbon
46 on a dry oxide surface. The inset shows a magnified view of the
blue end emission. FIG. 11B shows the same nanoribbon in a water
environment, under a quartz coverslip. The inset shows resultant
green emission. FIG. 11C shows the spectra of the two situations.
The large red shift of the empirical cutoff wavelength (from 483 nm
in air to approximately 570 nm in water) is caused by the decrease
in refractive index profile between the substrate and the cap
medium. The more homogeneous cladding index improves wave
confinement in the nanoribbon core. The effect was reversible by
evaporating the water.
[0147] As can be seen from FIG. 11C, the spectra of the guided PL
spectrum broadens to longer wavelengths when it is covered by pure
water. Such a red shift would be anomalous for a fiber with a
cladding of homogeneous refractive index, where one expects the
replacement of air (n=1) by water (n=1.33) to increase losses and
result in a blue shift of the mode cutoff. However, when a slab or
strip waveguide exists in an asymmetric cladding environment (that
is, when n.sub.waveguide>n.sub.subtrate>n.sub.cover), as it
does here, raising the index of the cover reduces its asymmetry
with the substrate and improves confinement.
[0148] Intuitively, the replacement of air (n=1) with water
(n=1.33) on three sides of a nanoribbon should increase its optical
loss and hinder waveguiding, especially for longer wavelengths. One
would expect a narrowing of the guided spectrum (a blue shift of
the cutoff wavelength). Instead, we found that the spectrum
broadens to the red and the end emission changes from blue in air
to green in water. This surprising result, which seems to suggest
that a smaller index profile between core and cladding results in
better, not poorer, confinement, is likely a consequence of the
smaller difference in refractive index between water and the
SiO.sub.2 substrate than between air and the substrate. The less
anisotropic water-silica cladding shifts the modal power nearer to
the center of the nanoribbon and thereby reduces overall radiative
loss. Ribbons that were too large to show a cutoff for PL were
unaffected by immersion in water.
[0149] FIG. 12A-12B demonstrates controlled manipulation of small
volume, substrate supported, liquid droplets. In this example a
droplet of approximately 5 .mu.L of 1,5-pentanediol was placed on a
silica substrate and then a commercial micromanipulator equipped
with an etched tungsten probe (tip diameter approximately 400 nm),
was utilized to dice the large droplet into small volumes. FIG. 12A
shows a dark-field image of various sized droplets of
1,5-pentanediol on a silicon substrate (with a 1 .mu.m thermal
oxide). The radii and corresponding volumes are displayed by each
droplet. FIG. 12B is a magnified dark-field image of smaller
droplets (<1 .mu.L). The radii and corresponding volumes (down
to approximately 20 .mu.L) are labeled on the dark-field image in
FIG. 12A. Even smaller volumes (<1 .mu.L) can be achieved with
this method as shown in FIG. 12B. By way of example, an alternative
method for producing small volumes is to utilize microfluidic
channels to mold the shape of the solution.
[0150] Ribbon waveguides can also sense molecules, proteins or
larger biological entities in solution by means of either an
emission or absorption mechanism as mentioned above. In the former,
a nanoribbon provides local excitation for fluorophores passing
through the cone of scattered light at its output end, and the
emission is collected by a fiber or microscope.
[0151] FIG. 13A-13D demonstrate a fluorescence scheme in which the
tip of a nanoribbon 48 is embedded in an approximately 3 pL to 5 pL
droplet of 1 mM Rhodamine 6G laser dye (R6G) in 1,5-pentanediol
(n=1.45). FIG. 13 shows fluorescence and absorbance detection of
R6G with a nanoribbon cavity. FIG. 13A is a fluorescence image of a
droplet of 1 mM R6G in 1,5-pentanediol excited by blue light from a
nanoribbon waveguide 48 (240 nm by 260 nm by 540 .mu.m). The
nanoribbon crosses the frame from upper left to lower right. A
notch filter was used to block the excitation light. The left inset
of FIG. 13A is a dark-field image showing the droplet and the
bottom half of the nanoribbon. The right inset of FIG. 13A is a
magnified view of the droplet emission, showing the light cone and
evanescent pumping of the dye along the nanoribbon length. FIG. 13B
shows the spectra taken of the droplet region (direct) and the
fluorescence coupled back into the nanoribbon (guided). The red
shift of the guided emission is a microcavity effect. FIG. 13C is a
dark-field image of the nanoribbon with a droplet deposited near
its middle (absorbance geometry). The nanoribbon was UV pumped on
one side of the droplet and probed on the other side, as indicated.
FIG. 13D shows the spectra of the guided PL without liquid present
and with droplets of pure 1,5-pentanediol and 1 mM R6G. The arrow
indicates the absorption maximum of R6G.
[0152] As can be seen, blue light (442 nm) launched into the far
end of the nanoribbon resulted in strong fluorescence from within
the droplet, where the R6G emission mapped out the spatial
intensity distribution of the waveguide output as a cone of light
(FIG. 13A and Inset). A fraction of this fluorescence was captured
by the nanoribbon cavity and guided back to its far end,
demonstrating that these waveguides are capable of routing signals
both from and to liquids. Spectra acquired from both ends of the
nanoribbon are shown in FIG. 13B. The guided fluorescence is
red-shifted and somewhat sculpted by its passage through the
nanoribbon. However, there is little trace of the heavy mode
imprinting evident in, for example, FIG. 17F discussed below.
[0153] FIG. 13B also shows strong fluorescence originating from the
segment of the nanoribbon wet by the droplet through capillary
action. Here, dye molecules in proximity to the nanoribbon surface
are excited in a subwavelength version of total internal reflection
fluorescence (TIRF). In normal TIRF, excitation of a macroscopic
waveguide (such as a microscope coverslip) generates an evanescent
field of light that decays exponentially with distance from the
waveguide surface, limiting the depth of excitation to a distance
of approximately 100 nm and enabling the local probing of
structures such as cell membranes. Because subwavelength fibers can
carry a larger fraction of their modal power outside of the core,
they enhance the intensity of this evanescent field and increase
its penetration depth into the surroundings, making proportionally
more power available to excite nearby molecules. Calculations
indicate that roughly thirteen to fifteen percent of the electric
field intensity exists outside of the nanoribbon for the wavelength
of light used in these tests. In this case, the radial field
intensity decays to ten percent of its maximum value at the center
of the waveguide by about 135 nm into the liquid solution. Since
TIRF detection sensitivity scales with the fractional power present
in the waveguide cladding, one-dimensional nanostructures are
promising waveguides for local fluorescence sensing using this
approach.
[0154] Another way that 1D nanostructures may be used for optical
detection in solution relies on producing an absorption spectrum of
molecules located on and near the nanoribbon surface. Absorbance
detection, while inherently less sensitive than fluorescence
methods, is applicable to a wider range of molecules and avoids the
complications of fluorescent tagging. White PL was launched down a
long nanoribbon (260 nm.times.240 nm.times.540 .mu.m) onto the
midpoint at which a droplet of 1 mM R6G (.alpha..sub.max=535 nm)
with a volume of approximately 1 pL was deposited (FIG. 13C). Dye
molecules in the droplet imprinted their absorption signature onto
the propagating PL wave (double-Gaussian beam), completely
quenching transmission through the nanoribbon around the R6G
absorption maximum (FIG. 13D). Considering the dye concentration,
droplet size and spatial extent of the evanescent field, we
estimate that less than forty attomoles of dye (.about.24 million
molecules) were probed in this experiment. In these tests we have
determined that sensitivities down to 50 .mu.M (.about.35,000
molecules) are easily attainable. Dye concentrations could be
detected as low as 1 .mu.M (24,000 molecules) using the same
nanoribbon and a comparable path length of approximately 50 .mu.m
(not shown). Since this absorbance approach also utilizes the
evanescent fraction of the guided field, smaller nanoribbons should
again provide greater sensitivity. Other options for improvement
include altering the cavity shape to increase the probe length (as
discussed below), functionalizing the nanoribbon surface for
selective biosensing and launching multiple wavelengths for the
simultaneous detection of analytes with different electronic
transitions. It should be appreciated that these subwavelength 1D
nanostructures can be integrated into microfluidic devices and
applied as flexible probes in the study of live cells.
[0155] A third way that subwavelength nanoribbons/wires can be used
for chemical/biological sensing relies on the surface enhanced
Raman spectroscopic (SERS) effect. Surface-enhanced Raman
scattering occurs when an analyte molecule is probed in proximity
to a metal surface (usually Cu, Ag or Au) that serves to massively
enhance the local electromagnetic field through resonance with the
surface plasmons of the metal. The resulting Raman signal of the
analyte can be enhanced by a factor of up to 10.sup.14, which
allows single-molecule sensing in many cases. The nanoribbons/wires
described here were fashioned into subwavelength SERS fibers by
decorating their surfaces with a high density of silver
nanoparticles. By exposing the nanoparticles-coated nanoribbon/wire
to an analyte solution while injecting monochromatic light down the
nanoribbon/wire, it is possible to detect the SERS signal of the
analyte molecule. This concept allows "fingerprint" identification
of analyte molecules based on their SERS vibrational signatures,
using a subwavelength waveguide for light introduction and
confinement.
[0156] FIG. 14A-14C illustrate the fingerprint identification
concept. FIG. 14A shows a schematic picture of this concept, while
FIG. 14B and FIG. 14C show an image of a nanoribbon (NR) coated
with 40 nm silver nanoparticles attached by exposing the nanoribbon
to a flowing nanoparticle solution. The particles are seen to
scatter the waveguided light very effectively. It is possible to
generate a SERS signal by then exposing the structure to an analyte
solution of interest. The device is reusable by simply dissolving
the Ag nanoparticles in an acidic solution (e.g., HNO.sub.3) and
then reintroducing fresh Ag particles.
[0157] The devices shown thus far all operate under single pass
geometries. Multi-pass structures would increase sampling lengths
and ultimately lead to a more sensitive spectrometer.
[0158] FIG. 15 shows a PL/dark-field image of two nanoribbons (NR1
and NR2) evanescently coupled at arrow 1. The top inset is a
magnified dark-field image of the coupled nanoribbons with a glycol
droplet designating where the analyte would sit in this
configuration. The bottom inset is a dark-field image of NR1 with
NR2 removed showing a coupled ring structure (junction--denoted at
arrow 2) that would serve as a multi-pass beam path in a
subwavelength optical spectrometer.
[0159] The figure illustrates that ring shapes can be easily
fashioned using our manipulation capabilities to create a
subwavelength cavity shape that would sample an analyte
repetitively. The glycol droplet (top inset) serves to identify
where the analyte would sit in this particular configuration. The
PL/dark-field image shows a two nanoribbon device evanescently
coupled (arrow 1 denotes the junction), illustrating the first step
to design a multi-pass spectrometer based on free-standing 1D
nanostructures. The bottom inset was taken after manipulating the
end of NR1 into a ring structure (arrow 2 denotes the junction)
showing the second step for creating a multi-cycle instrument.
Additional work is necessary to fully realize better sensitivity
from these advanced designs, but previous results on coupling
efficiencies suggest that up to an order of magnitude increase can
be obtained from a multi-pass geometry.
[0160] It should be appreciated that the fabrication of a practical
subwavelength fiber spectrometer as introduced above would benefit
from a more controlled flow-cell type microfluidic design in which
the sensing nanoribbon/wire is integrated with microfluidic
channels for solution introduction. An integrated device using a
poly-dimethylsiloxane (PDMS) stamp patterned with flow channels to
control analyte flow past an embedded nanoribbon/wire waveguide
have been built according to the present invention. With this
microfluidic design multiple analyte solutions can be pulsed past a
well-defined section of a sensing nanoribbon/wire, permitting reuse
of the sensor for biological and other liquid-based monitoring
uses.
[0161] FIG. 16A-16C illustrate microfluidic channels (MFC) of a
PDMS stamp bridged by multiple nanoribbons (NR). This is shown
schematically in FIG. 16A. FIG. 16B is an image showing
microfluidic channels in detail and FIG. 16C is an image showing
several nanoribbons bridging the microfluidic channels shown in
FIG. 16B. This microfluidic layout is important for the practical
use of these structures for fluorescence, absorbance and SERS
sensing.
[0162] It should be noted that the techniques and apparatus set
forth herein for chemically synthesized 1D semiconductor
nanostructures are entirely compatible with existing lithography
techniques. State-of-the-art electron beam and other lithography
methods currently offer better size control, reproducibility, and
processing speeds to produce subwavelength optical probes and
spectrometers than the serial approach discussed here. Future
experiments will include lithographically defined structures on
various support substrates to discern the limits of detection using
nanoscale optics.
[0163] In terms of present industrial efforts and interests in
small volume detection, NanoDrop.COPYRGT. Technologies has
developed a UV/Vis is spectrometer (ND-1000) based on patented
sample retention technology. The instrument is generally used to
detect 1 .mu.L to 2 .mu.L nucleic acid aliquots with a sample
detection limit of 2 ng/.mu.L (dsDNA). The path length for the Xe
flash lamp (220 nm to 750 nm) is held relatively fixed at 1 mm. The
major advantages of a subwavelength spectrometer over the
commercially available unit is smaller volume size (.about.10.sup.6
times smaller), shorter path lengths (.about.10 times shorter), and
possibly higher sensitivity with the advanced multi-pass
geometries.
8. Optical Routing with Nanoribbons and Nanowire Assemblies
[0164] The manipulation of optical energy in structures smaller
than the wavelength of light is key to the development of
integrated photonic devices for computing, communications and
sensing. Small groups of freestanding, chemically synthesized
nanoribbons and nanowires were fabricated into model structures
that illustrate how light is exchanged between subwavelength
cavities made of three different semiconductors. The strength of
the optical linkages formed when nanowires are brought into contact
depends both on their volume of interaction and angle of
intersection. Using simple coupling schemes, lasing nanowires can
launch coherent pulses of light through nanoribbon waveguides that
are up to several millimeters in length. Also, inter-wire coupling
losses are low enough to allow light to propagate across several
right-angle bends in a grid of crossed nanoribbons. The fraction of
the guided wave power traveling outside the nanowire/nanoribbon
cavities is utilized to link nanowires through space and to
separate colors within multi-ribbon networks. In addition, it was
found that nanoribbons function excellently as waveguides in liquid
media and provide a unique way to probe molecules in solution or in
proximity to the waveguide surface. Our results lay the groundwork
for photonic devices based on assemblies of active and passive
nanowire elements and presage the use of nanowire waveguides in
microfluidics and biology.
8.1 Example 3
[0165] SnO.sub.2 nanoribbons were synthesized by the chemical vapor
transport of SnO at 1100.degree. C. in flowing argon. ZnO nanowires
were grown as epitaxial arrays on sapphire substrates by the
oxidation of metallic zinc at 800.degree. C., using gold as a
catalyst. GaN nanowires were made by the chemical vapor transport
of gallium in a NH.sub.3/H.sub.2 mixture at 900.degree. C., with
nickel as the catalyst. The SnO.sub.2 nanoribbons were dry
transferred en masse to oxidized silicon substrates (600 nm
SiO.sub.2, Silicon Sense Inc.). A triple-axis micromanipulator
tipped with a tungsten probe (.about.400 nm tip diameter) was used
to remove individual ZnO and GaN nanowires (chosen by their PL
spectra) from their growth substrates and then deposit them with
the nanoribbons.
8.2 Example 4
[0166] Nanoribbons and nanowires were manipulated with the probe
under a dark-field microscope. A HeCd laser provided continuous
wave (CW) resonant illumination (325 nm), while the fourth-harmonic
of a Nd:YAG laser (266 nm, 8 nm, 10 Hz) was used for pulsed
pumping. Laser diodes (652 nm and 532 nm) and the HeCd laser (442
nm) supplied visible light for the filtering and fluorescence
demonstrations. The lasers were focused to a beam diameter of
approximately 50 .mu.m, giving a CW power density of approximately
175 W/cm.sup.2 and a pulsed energy density of approximately 10
.mu.J/cm.sup.2. Spectra were acquired with a fiber-coupled
spectrometer (gratings at 150 and 1200 grooves/mm, SpectraPro 300i,
Roper Scientific) and liquid N.sub.2-cooled CCD setup.
Black-and-white and color images were recorded with two
microscope-mounted CCD cameras (CoolSnap fx and CoolSnap cf,
Photometrics).
[0167] Many of the nanoribbons/wires described herein operated as
single-mode fibers for some of the experimental wavelengths, while
others were multi-mode. For reference, the approximate single-mode
cutoff diameters of a cylindrical step-index fiber in air are 140
nm (.lamda.=365 nm) and 265 nm (.lamda.=600 nm) for SnO.sub.2, 112
nm (.lamda.=365 nm) for GaN, and 140 nm (.lamda.=380 nm) and 220 nm
(.lamda.=510 nm) for ZnO.
[0168] In the liquid experiments, large droplets (.about.5 .mu.L)
of water or various alcohols were transferred to the oxide surface
by pipette. The solvent droplets were then diced into smaller
volumes (as small as 100 fL) and positioned on the surface using
the manipulator.
9. Nanoribbon/Wire Sizes Determined with a (SEM)
[0169] FIG. 17A-F and FIG. 18A-B illustrate several implementations
utilizing a single nanoribbon in various combinations with GaN and
ZnO nanowires.
[0170] FIG. 17A-F illustrate the routing of GaN PL and lasing
emission. FIG. 17A is a dark-field optical image of a coupled GaN
nanowire 50 and SnO.sub.2 nanoribbon 52. The label A denotes the
location of the junction. FIG. 17B shows direct excitation of the
SnO.sub.2 nanoribbon at location B generates white PL that is
guided to the ends of the SnO.sub.2 cavity. Some of the light is
scattered by a large particle found at C. The inset in FIG. 17B is
a magnified view of the bottom emission spot. FIG. 17C is a
magnified view of the junction area. The inset in FIG. 17C is a SEM
image showing that the two structures are staggered over 9 .mu.m
and touch for approximately 2 .mu.m. FIG. 17D shows direct CW
excitation of the GaN nanowire generates UV band-edge emission at
365 nm and a small amount of visible defect emission at 650 nm. The
cavity is too thin to permit the confinement of red light, but
(Inset) a UV camera detects strong waveguiding of the UV PL. FIG.
17E is an optical image of the routing of UV laser pulses from
nanowire to nanoribbon. Here, the GaN cavity was pumped above its
lasing threshold by a pulsed 266 nm source (itself invisible to
this detector). FIG. 17F shows spectra comparing the GaN PL and
lasing emission before and after passage through the nanoribbon
cavity. The broad pseudo-Gaussian spontaneous emission peak (top)
is broken into a series of sharp modes during its transit through
the nanoribbon (WG PL). Likewise, the lasing emission at moderate
pump power, which shows multiple modes (GaN lasing), is severely
modulated by the mode structure of the SnO.sub.2 cavity (bottom).
Spectra are normalized and offset for clarity.
[0171] As can be seen, FIG. 17A shows a GaN nanowire (130 nm by 65
.mu.m) that has been coupled to a SnO.sub.2 nanoribbon (240 nm by
260 nm by 460 .mu.m) with the micromanipulator. The magnified SEM
view of the GaN--SnO.sub.2 junction (Inset, FIG. 17B) indicates
that the two structures are in physical contact over an interaction
length of approximately 2 .mu.m. This staggered-bonded
configuration provides good optical coupling between the cavities
and some degree of inter-wire adhesion (via electrostatic forces),
which aids in the construction of multi-wire networks. Butt-end
coupling is also effective, and it is possible for us to detect the
transfer of light between nanowire cavities that are weakly coupled
across an air gap of up to several hundred nanometers (not shown).
If two nanoribbons are crossed instead of staggered, the coupling
losses decrease with shallower intersection angles, which has also
been observed recently for crossed CdS nanowires.
[0172] To demonstrate the routing of continuous wave light, we
excited the GaN nanowire with the focused beam of a HeCd laser
operating at 325 nm. Band-edge PL from the GaN cavity was channeled
through the SnO.sub.2 nanoribbon to emerge primarily at its far
end. A fraction of the light was also scattered by imperfections
along the length of the nanoribbon (i.e., attached particles or
macroscopic step edges). Far-field spectra collected from the
output end of the nanoribbon (FIG. 17F) show that the
quasi-Gaussian PL band of GaN is imprinted with the mode structure
of the SnO.sub.2 cavity during its transit. This mode structure is
not longitudinal (Fabry-Perot) in nature, as it is for shorter
nanowires; instead, it is a complex interference pattern dependent
on nanoribbon shape and cross-sectional dimensions, among other
factors.
[0173] FIG. 18A-18B illustrate that it is possible to
simultaneously guide the output of two (or more) nanolasers by
coupling multiple ZnO and GaN nanowires to the same nanoribbon,
opening up the possibility of performing nonlinear wave mixing
within single nanocavities. FIG. 18A is a dark-field image of a GaN
nanowire 54 and a ZnO nanowire 56 coupled to the same nanoribbon
58, the scale bar is 10 .mu.m. FIG. 18B shows the spectrum of
guided light collected at the far end of the nanoribbon when both
nanowires were pumped above their lasing thresholds by the same
train of optical pulses. The nanoribbon is the same used in FIG.
13A-13D and FIG. 17A-17F.
[0174] It should be appreciated that in contrast to their
continuous wave emission, the pulsed emission of ZnO and GaN is
nearly devoid of visible PL since the defect bands experience no
gain. This is experimental verification that coherent optical
pulses can be transferred between nanowires and steered hundreds of
micrometers from their source. By utilizing high frequency
electrical pumping these nanowire laser/waveguide combinations
could be used to transduce and shuttle packets of electro-optical
information within future computing and communications devices.
[0175] FIG. 19A-19B illustrate GaN nanowire lasing. In FIG. 19A is
shown a series of emission spectra at different pump fluence for an
isolated GaN nanowire with a diameter of 150 nm and length of 45
.mu.m. The inset in FIG. 19A shows the PL spectrum. FIG. 19B shows
the energy curve for the same nanowire. Typical thresholds for GaN
NW lasing were 5 .mu.J to 15 .mu.J cm.sup.-2. The inset in FIG. 19B
is an image of lasing emission from a different GaN nanowire,
showing its pronounced spatial pattern. By pumping the GaN nanowire
above its lasing threshold (.about.5 .mu.j/cm.sup.2) with pulsed UV
excitation, single optical pulses were sent out from the nanowire
laser through the nanoribbon waveguide (FIG. 17E). The spectrum of
several thousand accumulated pulses (FIG. 17F) shows a series of
sharp modes (FWHM=0.8 nm) slightly red-shifted from the band edge
of GaN. These sharp modes illustrate the Fabry-Perot type lasing
modes of the GaN nanowire resonator, whose intensity is modulated
by the nanoribbon cavity. Similar results have been with junctions
between nanoribbons and lasing ZnO nanowires.
[0176] FIG. 20A-E and FIG. 21 illustrate that since diffraction
losses in a subwavelength cavity increase markedly with wavelength,
a nanoribbon waveguide preferentially confines the bluer portion of
any non-monochromatic beam. As a result, nanoribbons act as
short-pass filters with cutoff wavelengths that are determined by
their cross-sectional dimensions and overall length.
[0177] In FIG. 20A-E color filtering is shown in a nanoribbon
network 60. FIG. 20A is a dark-field image of a four-ribbon
assembly as it guides white PL generated at the pump spot (left)
and separates it into a different color at the end of each
nanoribbon (right), the scale bar is 50 .mu.m. FIG. 20B depicts a
magnified view of the emission region. The branch nanoribbons 1-3
in FIG. 20B emitted green, aqua and blue light because of their
progressively smaller cross-sections (350 nm by 140 nm, 260 nm by
175 nm and 210 nm by 135 nm, respectively). Their 50% cutoff
wavelengths were determined by near-field scanning optical
microscopy (NSOM) to be 543 nm, 502 nm and 478 nm. The stem
nanoribbon is 260 nm by 240 nm by 390 .mu.m. FIG. 20C shows that
non-resonant blue light is transmitted to the end of all four
nanoribbons, while FIG. 20D shows that green light is much more
strongly guided by nanoribbon 1 than by nanoribbon 3 and FIG. 20E
shows that red light is filtered out by all three branches. The
scale bar is 20 .mu.m.
[0178] As can be seen from FIG. 20, a simple network was assembled
comprising four nanoribbons of different sizes to show how such a
structure may be used to separate colors. When excited at 325 nm,
the large nanoribbon that formed the stem of network 60 emitted
white light composed of two broad SnO.sub.2 PL bands centered at
495 nm and 590 nm, as can be seen from FIG. 21 which is a typical
PL spectrum of a SnO.sub.2 nanoribbon showing its two defect bands.
Varying amounts of stem emission then flows into the three shorter
and consecutively thinner branch nanoribbons, separating the white
light into green, aqua and blue components (ribbons 1-3).
Alternatively, when monochromatic red light was launched into only
the stem, then the stem nanoribbon lit up, while green light was
guided strongly (weakly) by the largest (smallest) branch and blue
light passed through all three branches as well as the stem (FIG.
20B-20D). Although this color filtering effect works only in
short-pass mode, and thus cannot, for instance, isolate the pure
red component of a white beam, it may prove useful in such tasks as
removing visible contamination from UV pulses or providing local
excitation for fluorophores with narrow absorption bands, such as
quantum dots.
[0179] FIG. 22 illustrates testing the limits of inter-cavity
optical coupling, in which four nanoribbons were assembled into a
rectangular grid (46 .mu.m long by .about.25 .mu.m wide) featuring
X-junction vertices with small contact areas (<0.15 .mu.m.sup.2)
(FIG. 22A and Inset). FIG. 22A is a dark-field image of the
four-ribbon structure, with the input channel extending off the
frame to the right and the output channels labeled 1-7. The
nanoribbons vary in size from 300-400 nm on a side. (FIG. 22A and
Inset). A SEM image of the junction is shown at the lower right
vertex. FIG. 22B is a PL image as the input channel is pumped at
325 nm. Light is guided to the seven output ends with different
intensities and colors as described below.
[0180] The structure was designed with one long channel for light
input and seven short output channels that could be monitored
simultaneously. As shown in FIG. 22B, direct excitation of the
input channel triggered emission from all seven of the nanoribbon
outputs, with the following intensity distribution:
1>>6>4.apprxeq.7>3>5>2. This is exactly the
sequence one would expect after considering the trajectory of the
incoming light and the intensity of scattering at the four
nanoribbon-ribbon junctions. The light trajectory is important here
since the low reflectivity of their end facets makes nanoribbons
poor resonators (with an ideal finesse of .about.1.3). As such,
most photons do not make multiple passes and light flow is highly
directional. The right-angle intersections present significant
obstacles to inter-cavity waveguiding by total internal reflection.
At the same time, they act as quasi-isotropic scatterers that feed
light between nanoribbons. Nanoribbon-to-nanoribbon losses,
although nearly maximized in this geometry, are still low enough
for the activation of channels 2 and 3, which require photons to
negotiate two right-angle junctions and transit three separate
cavities. When a ZnO nanowire laser was added to the input channel
and used to launch light into the grid, emission was detected from
all channels but 2 and 3. In this case the number of injected
photons was simply too small to illuminate the parallel nanoribbon.
Nanowire grids have already been employed to implement rudimentary
electronic logic. Integrated optical logic and all-optical switches
present exciting prospects, and these results illustrate that grids
of nanowires should be capable of routing signals for such
tasks.
[0181] Due to their high refractive indices (n.gtoreq.2), the
nanoribbons and nanowires discussed here function well as
waveguides in water and other liquids. This is a considerable
advantage over subwavelength silica waveguides, which cannot
efficiently confine visible light in liquids because of a low
dielectric contrast (n.sub.silica.apprxeq.1.45). Waveguiding in
liquids is especially important for integrated on-chip chemical
analysis and biological spectroscopy in which small excitation and
detection volumes are required.
[0182] As can be seen, chemically synthesized nanoribbon and
nanowire waveguides have two unique and potentially useful features
for subwavelength photonics applications. First, nanowires push
subwavelength optical fibers beyond silica. The scores of materials
that can now be made in nanowire form include active, passive,
nonlinear and semiconducting inorganic crystals, as well as a wide
variety of polymers. Simultaneous photon, charge carrier and spin
manipulation is possible within and between nanowires of different
compositions. Also, many of these materials have higher refractive
indices than silica-based glasses, permitting light of a given
wavelength to be confined within thinner structures for denser
integration. This enables waveguiding in liquids and makes it
possible to extend subwavelength guiding to telecommunications
wavelengths using, for example, approximately 300 nm diameter Si or
GaP nanowires. Second, nanowires are freestanding, mechanically
flexible elements that can be manipulated on surfaces or used as
mobile probes in fluids. As such, they offer a type of versatility
difficult to achieve with lithographically-defined structures that
are permanently affixed to their substrates.
[0183] The disadvantages of nanowire photonics include (i) the
paucity of parallel assembly methods for accurately arranging large
groups of nanowires into useful structures; (ii) relatively high
inter-wire coupling losses compared to monolithic waveguides formed
by lithography (coupling losses could be greatly reduced if
branched, multi-component nanowires were developed to replace the
staggered or crossed nanowire cavities used here); (iii) the lesser
geometric perfection of nanowire assemblies relative to the precise
shapes and sizes definable with lithography. Geometric imprecision
introduces some uncertainty in the resulting light propagation and
adds complexity to nanowire experiment/theory comparisons. However,
despite these limitations, nanowires and their assemblies provide
an important new platform for photonics studies and applications
that is only beginning to be investigated.
[0184] It will be appreciated that the subwavelength waveguide
described herein can be used as a functional element in photonic
circuits such as optical networks, optical filters, optical
directional couplers, emitter-waveguide-detector junctions, optical
probes, optical sensors, optical routers, optical junctions,
optical modulators, optical Y-junctions, optical branch networks,
Mach-Zehnder interferometers, optical ring oscillators, nanolasers,
optical phase shifters, fluidic sensors, fluidic probes,
microfluidic devices, optical spectrometers, and optical crossbar
grids. Those skilled in the art will also appreciate that the
nanostructures described herein can be fabricated and incorporated
into devices, systems and structures using various techniques known
in the art. Additionally, reference is made to U.S. Pat. No.
6,882,051, entitled "NANOWIRES, NANOSTRUCTURES AND DEVICES
FABRICATED THEREFROM" issued on Apr. 19, 2005, which is
incorporated herein by reference in its entirety, and to U.S.
Patent Application Publication No. US 2004/0131537 A1, entitled
"FUNCTIONAL BIMORPH COMPOSITE NANOTAPES AND METHODS OF FABRICATION"
published on Jul. 8, 2004, also incorporated herein by reference in
its entirety.
10. Optical Sensors
[0185] As discussed above, subwavelength waveguides can be utilized
as optical probes and sensors. In this section a novel optical
sensing platform is described that utilizes the evanescent field of
a single-crystalline nanoribbon waveguide to optically characterize
sub-picoliter volumes of solution, for example performing
absorbance, fluorescence and surface enhanced Raman spectroscopy
(SERS). Chemical specificity of SERS was obtained by decorating
(configuring) the waveguide with metallic nanoparticles, in this
case silver nanocubes, to enhance the field around the nanoribbon.
The waveguide sensors exhibited excellent chemical resistance and
can withstand cleaning cycles in strong acid, making the devices
reusable. These results open the path to our engineering of
hand-held, photonic sensors capable of detecting and/or identifying
chemical species in solution. The validity of this embodiment has
been demonstrated by directly exciting molecules (i.e., absorbance
and/or fluorescence) with the evanescent field or by scattering
light off metallic nanoparticles immersed in the evanescent field
to enhance local Raman modes. The nanowire optical sensing platform
described herein complements nanowire field effect sensors with an
ability to monitor optical attenuation across the wire element.
However, the use of photons instead of electrons allows optical
spectroscopy to be carried out on the analyte.
[0186] To simplify material manufacturing a plurality of optical
waveguides were chemically synthesized having a sub-optical
diameter, such as with sub-200 nm diameters, to expose a
substantial amount of the guided optical intensity to the
surrounding matrix. This field was strong enough to optically
interrogate molecular species without disrupting the operation of
the waveguide.
[0187] FIG. 23A-23C illustrate a device layout of an evanescent
field sensor 100 according to the invention. FIG. 23A is an image
of a device showing fluidic structures comprising PDMS microfluidic
sensing channels 102 and a supporting quartz coverslip 104 (shown
in FIG. 23C). The lower inset is a magnified dark-field image of an
example depicting five microfluidic sensing channels 102. The upper
inset is a schematic diagram showing the inlet/outlet ports 106a,
106b and location of the waveguides 108. FIG. 23B is a
dark-field/luminescence image of a single SnO.sub.2 waveguide
bridging two sensing channels. The waveguide is optically pumped
outside the field of view (bottom left) and the guided emission is
routed to the end facet at top right. FIG. 23C is a schematic
diagram of the device orientation once mounted to the microscope in
relation to the objective 110, for acquiring the corresponding
images shown in FIG. 23A-23B.
[0188] FIG. 24A-24H illustrate use of evanescent wave sensors in
absorbance and fluorescence modes. FIG. 24A is a diagram showing
the absorbance geometry, and shows a nanoribbon 108 in relation to
an evanescent field 110 and molecules 112. FIG. 24B shows raw
waveguided fluorescence spectra before (clean PL--thin black line)
and during (sense PL--dotted line) the flow of a 3 mM solution of
EITC through a single sensing channel. The resulting absorption
spectrum is shown in the lower thick black line. The inset shows
cycling the device through multiple sensing cycles using pH12 water
to clean the waveguide. FIG. 24C shows the absorption spectra of
four EITC solutions of different concentration. The inset shows
peak absorbance versus EITC concentration showing the linear
response of the sensor in this range. FIG. 24D is a comparison of
thin (d<150 nm) and thick (d>200 nm) nanoribbon waveguides.
The upper and lower solid traces are the raw waveguide absorption
data and the upper and lower thin dashed traces (normalized) were
taken with a conventional UV/Vis is spectrometer. FIG. 24E shows
overlaid absorbance and fluorescence spectra of a 1.84 mM EITC
solution using the same waveguide. The insets are photoluminescence
images of a sensor in the presence of EITC (upper image) and water
(lower image). FIG. 24F-24H are a series of photoluminescence
images captured at 5 ms (187 Hz) of .lamda.-DNA-YOYO1 molecules
flowing past a sensor. In this case 442 nm light (<10 nW) is
guided from right to left, wherein the arrow in FIG. 24G denotes a
single molecule passing through the evanescent field.
[0189] FIG. 25A-25D illustrates dielectric scattering and
refractive index sensing with silver nanoparticles. FIG. 25A is a
dark-field image of a SnO.sub.2 waveguide bridging a single sensing
channel. The arrows denotes the direction of the light and fluid
flows. FIG. 25B shows scattering images recording Ag nanoparticles
(NPs) as they stick to the waveguide surface. White light was
launched through the waveguide from left to right by pumping its
end (outside the field of view) with 325 nm light. From top to
bottom, the time elapsed is approximately 30 seconds. FIG. 25C
shows scattering images of a metal-decorated waveguide upon
introduction of a 1:1 HCl/HNO.sub.3:H.sub.2O solution. From top to
bottom, the time elapsed for cleaning is 30 seconds. FIG. 25D shows
raw scattering spectra collected from the channel as various
liquids flow across the waveguide. The upper inset shows scattering
intensity versus the index of refraction showing the expected
linear response. The inverse dependence of intensity on the index
of refraction is caused by the increase in waveguide loss as the
cladding index increases. The insets on the lower right show
scattering images taken from the adsorbed Ag nanoparticles in
ethanol (n=1.36) and glycol (n=1.43).
[0190] FIG. 26A-26C illustrate a nanoribbon evanescent wave SERS
sensor. FIG. 26A is a schematic diagram of the sensing scheme.
Analyte molecules 112 in close proximity to a metal-decorated (by,
for example, Ag nanoparticles 114) nanoribbon are excited by the
surrounding evanescent field and show amplified Raman scattering,
which is then detected with the microscope objective 110 (FIG.
23C). FIG. 26B shows resonant SERS spectra of 100 mM Rhodamine 6G.
Light was delivered to the particles by direct excitation (red
line) or by the evanescent field of the waveguide (SERS R6G WG
traces). Large (d.about.500 nm) and small (d.about.150 nm)
waveguides yielded identical spectra. The background (dashed trace,
second from bottom) was acquired with the beam positioned off the
end facet of the waveguide. A Raman spectrum of PDMS (bottom solid
trace) verifies that the background results from PDMS. The insets
show scattering images taken of the large (top) and small (bottom)
waveguides. FIG. 26C shows nonresonant SERS of bound dodecanethiol.
Direct (Thiol Confocal--top line) and waveguide-excited (Thiol
WG--second line from top) SERS spectra both show the distinct C-C
stretching modes of the thiol ligands at 1080 and 1122 cm.sup.-1.
Background (third line from top) and PDMS Raman (bottom line)
spectra are provided for clarity.
[0191] FIG. 27A-27B illustrate absorbance spectra of a positively
charged dye, rhodamine 6G. FIG. 27A shows fluorescence from a
SnO.sub.2 waveguide after traveling through the dye. The attenuated
portion of the fluorescence increases as a function of time due to
multilayer formation of the dye on the negatively charged SnO.sub.2
surface. FIG. 27B shows absorption spectra from the same series
shown in FIG. 27A along with a normalized absorption spectrum from
a conventional UV/Vis is spectrometer. The inset shows peak
absorbance plotted verses time. The linear fit compared to the
Langmuir isotherm curve suggests multilayers of the cationic dye
are forming on the surface of the waveguide.
[0192] FIG. 28A is a schematic of two coupled nanoribbons for
multi-pass absorption showing two possible locations for the
analyte. The first ribbon 108a (NR 1) is shaped into a ring 116 and
a second ribbon 108b (NR 2) is evanescently coupled so a portion of
the signal is routed to a detector 116. FIG. 28B-28C are images of
two SnO.sub.2 nanoribbon waveguides physically manipulated into the
structure depicted in FIG. 28A. The inset is an image taken after
placing a small droplet of 1,5-pentanediol (PD) on the sensing
region of the ring. FIG. 28D shows spectra recorded on the output
end of NR 2, in FIG. 27B, after placing PD droplets, loaded with
various concentrations of rhodamine 6G dye, on NR 1. Spectra were
taken approximately 5 s after droplet deposition.
10.1 Example 1
[0193] Tin dioxide (SnO.sub.2) nanoribbons were synthesized through
a chemical vapor transport process. An alumina boat filled with tin
monoxide powder was heated (1100.degree. C.) in an alumina tube
under a continuous flow of argon (300 torr) for approximately 2
hours. After removing the boat from the tube furnace, the
nanoribbons were deposited on a clean glass substrate for optical
characterization (see below). For surface enhanced Raman
spectroscopy (SERS) detection, silver nanocrystals were prepared
using a modified polyol process in which silver nitrate is reduced
in a solution of 1,5-pentanediol (.about.190.degree. C.) in the
presence of a capping polymer.
[0194] These tests were performed with an upright dark-field
microscope operating in reflection mode. Monochromatic laser light
was focused onto the sample at a 35.degree. angle normal to the
substrate. Broadband light (FWHM>200 nm) was generated in the
waveguide by exciting the SnO.sub.2 nanoribbon with the 325 nm line
of a continuous-wave HeCd laser (Melles Griot, Irvine, Calif.). The
broad luminescence of the waveguide was used for the absorbance
measurements by detecting the guided light with and without the
analyte present. Fluorescence and SERS spectra were captured by
focusing either a 442 nm (HeCd) or 532 nm (CW diode) laser spot on
one of the end facets of the waveguide. The signal was collected by
a 50.times. objective (Nikon, 0.55 NA) and routed through a fiber
to a spectrometer (150 grooves/mm grating, SpectraPro 300i, Roper
Scientific, Trenton, N.J.) equipped with a liquid nitrogen cooled
CCD. Images were captured either with a digital color camera
(CoolSnap cf, Photometrics, Tucson, Ariz.) or an EMCCD camera
(iXon, Andor Technology, Belfast, Northern Ireland).
[0195] Microfluidic flow cells were cast from polydimethylsiloxane
(PDMS) using standard lithography. A silicon master containing five
parallel channels (channel lengths of 1.5 mm and inter-channel
separation of 100 .mu.m) was prepared with 50 .mu.m wide.times.50
.mu.m deep channels. After casting and curing PDMS on the silicon
master, the stamps were removed from the master, cleaned with
ethanol and dried. To increase exposure of the analyte to the
surface area of the cavity, the waveguides were deposited in a wet
PDMS layer (.about.5 .mu.m thick) inked on the structured side of
the stamp. This also ensured complete sealing of the channels with
the quartz substrate after PDMS curing and eliminated capillary
leakage between adjacent channels. With the PDMS layer uncured,
nanoribbons were placed across the channels using a
micromanipulator (Marzhauser Wetzlar, Wetzlar-Steindorf, Germany)
equipped with an etched tungsten probe. Only ribbons with lengths
greater than 350 .mu.m were used in the devices. The stamp was then
immediately bonded to a quartz substrate, giving the final device
architecture shown in FIG. 23A after curing the wet layer. The
sharp waveguide/PDMS interfaces can be seen in FIG. 23B which is a
dark-field/luminescence image of a single SnO.sub.2 waveguide
bridging two sensing channels. The waveguide is optically pumped
outside the field of view (bottom left) and the guided emission is
routed to the end facet at top right. Referring to FIG. 23C, the
optical tests were carried out with the device oriented
horizontally in the microscope with the quartz substrate facing the
collection objective. Analyte solutions were pulled through the
channels using a syringe pump operating at flow rates of 0.75-3
mL/h.
[0196] Unless specified, waveguides were chosen for sensing if
their single-mode cut-off wavelengths were 550 nm or shorter.
Waveguides were screened optically on a silica surface by pumping
one of the ends of the nanoribbon with above band gap light
(E.sub.g=3.6 eV) from the HeCd laser (3.81 eV), and collecting the
waveguided defect emission in the far-field at the opposite end of
the ribbon. The empirical cut-off wavelength (.about.550 nm) for
the waveguides was determined by identifying the 50% transmission
point (inflection point) in the collected emission spectrum. Using
the waveguide parameter for a cylindrical fiber, the single-mode
cut-off wavelength takes on the form
.mu.=d.pi./2.405(n.sub.co.sup.2-n.sub.cl.sup.2).sup.112 where d is
the single-mode cut-off diameter of the waveguide and n.sub.co and
n.sub.cl are the refractive indices of the waveguide core (n=2.1)
and cladding (n=1), respectively. Although the nanoribbons have
rectangular cross-sections, it was found that this generalized
expression yields cut-off diameters on the order of 200 nm, in good
agreement (d=150-200 nm) with the dimensions of the ribbons
used.
10.2 Example 2
[0197] In the following example SnO.sub.2 nanoribbons were utilized
as the passive optical components in the devices as their high
index of refraction allows efficient waveguiding through the
microfluidic devices and analytes. In addition, the superb chemical
and mechanical properties of these nanoribbons allows them to
withstand harsh cleaning conditions.
[0198] The first spectroscopic experiment performed with the
optical waveguides was the acquisition of an absorption spectrum.
This is achieved by generating the featureless defect emission on
one side of the sensing channel and collected on the opposing side
after it is guided through the analyte (see FIG. 23B). The dye
chosen for the absorption measurements was eosin-5-isothiocyanate
(EITC) (Molecular Probes) which is a common amine-reactive probe
used to conjugate proteins or amine-modified oligonucleotides. EITC
solutions were prepared in water (pH 12) with concentrations
ranging from 0.75 mM to 3 mM. The solutions were pulled into the
channel and sensed under continuous flow. The initial waveguided
intensity (I.sub.o) is compared to the sensing intensity (I.sub.s)
to determine the absorbance (A) of the solution according to A=log
[I.sub.o/I.sub.s] (FIG. 24B). The attenuated fraction of I.sub.s is
directly related to the absorbance of the analyte species. The
waveguides are cleaned (FIG. 24B-inset) by pulsing pure solvent (in
this case pH12 water) into the channel. The concentration
dependence is linear with a molar absorptivity (.di-elect cons.) of
31,000 M-.sup.1cm.sup.-1, which is approximately 3.times. lower
than estimated values for EITC in 0.01 M NaOH. Although the surface
of the SnO.sub.2 nanoribbon is negatively charged at this pH
(isoelectric point .about.5) and EITC carries a net-negative
charge, a slight enrichment of the analyte near the waveguide/PDMS
interface may occur due to Van der Waals forces between the EITC
molecules and polymer. This interaction would cause a lower molar
absorptivity as seen in the EITC data. When cationic dyes such as
Rhodamine 6G are pulled through the flow cells, multi-layers form
on the surface of these waveguides (see FIG. 27).
[0199] As the SnO.sub.2 defect emission travels through the sensing
region it is attenuated according to
I.sub.s=I.sub.oexp.sup..di-elect
cons..kappa.Lc-I.sub.o(NA.sub.o.sup.2/NA.sup.2) where .di-elect
cons. is the molar absorptivity of the analyte (M.sup.-1cm.sup.-1),
.kappa. is the percent power in the evanescent field, L is the
sensor length (cm), c is the analyte concentration, and NA and
NA.sub.o(NA=(n.sub.co.sup.2-n.sub.cl.sup.2).sup.1/2) are the
numerical apertures of the waveguide with and without the analyte,
respectively. This is a simplified expression that neglects
chemical enrichment around the SnO.sub.2 ribbon, the shape of the
cavity and the dispersion of the field penetration; however, it
gives estimates ranging from 15% to 30% for the power available in
the evanescent field for the waveguide dimensions used here. These
results closely correspond to calculations describing the percent
power in the core (.eta.) of a step index fiber which uses the
function .eta.=1-[5.784exp(-2/V)/V.sup.3] where
V=.pi.d/.lamda.(n.sub.co.sup.2-n.sub.cl.sup.2).sup.1/2 (d is the
diameter of the fiber). For example, a 200 nm fiber (n.sub.co=2.1)
guiding 500 nm light in water (n.sub.co=1.33) contains
approximately 80% of the power within the core and approximately
20% in the evanescent field. The penetration depth (defined where
the field intensity decays to 10% of the core power) is calculated
to be about 125 nm, leading to a probe volume of less than 10 fL
for a 50 .mu.m path length. With some simple modifications to path
length, index of analyte and cavity size, it should be possible to
reach <100 attoliter (10.sup.-18 L) probe volumes. The detection
limit for absorption with a single pass through the analyte is
approximately 0.3 mM, but improvements should be possible by
utilizing multi-pass ring geometries (see FIG. 28). Initial
experiments were performed by molding ribbons into ring structures
on a silica surface and placing free-standing analyte droplets on
the waveguide. With this scheme, however, it is difficult to
control the path length of the sensing regions due to capillary
wetting of the analyte along the waveguide surface. To lower the
detection limit of absorption, and control the size of the sensing
region, it will be important to integrate such designs within a
microfluidic flow cell.
[0200] Since photons emitted near the ribbon surface can be
recaptured by the waveguide, the resulting absorption line shape
can be skewed. This causes an artificial decrease in the calculated
absorption for longer wavelengths (FIG. 24D--top trace). We observe
such an artifact only in thinner waveguides (diameters <150 nm),
which carry a larger percentage of the guided field intensity in
the cladding. For thicker nanoribbons (diameters >200 nm) the
number of photons coupled back into the waveguide is reduced, but
the absorption linewidth is slightly larger (FIG. 24D--bottom
trace). This can be explained by the variation in penetration depth
of the evanescent field as a function of wavelength. Longer
wavelengths (in this case >525 nm) show an increase in the
absorption because they penetrate deeper into the solution. For all
ribbons sizes, however, the peak maximum matches that from a
commercial photospectrometer to within approximately 2 nm. More
accurate peak shapes can be obtained by using expressions that
account for the amount of light accessible to the analyte at
different wavelengths as well as the photon flux that is recaptured
by the waveguide.
[0201] To characterize the fluorescent signals produced by the
analyte, monochromatic light is launched down the cavity to excite
molecules passing through the evanescent field. This mode of
detection is analogous to total internal reflection fluorescence
(TIRF); however, a more intense optical field resides near the
core-cladding interface of a sub-200 nm nanoribbon. As with TIRF,
no quenching or surface effects was observed which alter the
fluorescence spectra (FIG. 24E) of the molecules excited by the
evanescent field. It should be noted that this is often a drawback
of metal-coated surfaces which can influence the spectrum of
fluorophores or macromolecules either through quenching or effects
caused by molecules adsorbing to the metal surface. The
fluorescence image in FIG. 24E (upper image) illustrates a typical
intensity gradient along the length of a nanoribbon as 532 nm light
travels though it from left to right. The decay in fluorescence
intensity is a result of analyte absorption as the guided light
moves across the sensing channel. Less than 100 nW is confined
within the core of the waveguide in this example. The signal
appears line-like because the solution is too concentrated to
permit single fluorophores to be distinguished. However, individual
molecules can be observed if nanomolar solutions are used. This is
demonstrated in the case of YOYO-1 labeled .lamda.-DNA (FIG.
24F-H). Here, portions of the waveguide appear to have higher
concentrations of DNA near one of the edges. The likely explanation
for this is the local interactions between the DNA strands and the
SnO.sub.2 surface causing a slight enrichment of the DNA
molecules.
[0202] Light can be extracted from the waveguide by immersing
particles with large dielectric constants in the evanescent field.
This becomes important for ultra sensitive detection with a
subwavelength fiber because a single particle can scatter a large
percentage (5% to 10%) of the confined optical energy. We found
that silver nanocubes 50 nm in diameter readily adsorbed to the
waveguide surface and intensely scattered waveguided light (FIG.
25B). In general, strong scattering is observed when any particle
entering the evanescent field has a larger index of refraction than
the waveguide. For most waveguides used here (d.about.150 nm to 200
nm), complete attenuation of the waveguided light occurred when
approximately 10-15 Ag nanoparticles attach to the surface (FIG.
25B--lower image). The scattering intensity from these immobilized
nanocubes can be used to quantify the index of refraction
surrounding the nanoribbon. This is demonstrated in FIG. 25D for
three different refractive index media (air n=1, ethanol n=1.36,
and glycol n=1.43). As expected the scattering intensity decreases
linearly with increasing index of refraction. To remove the metal
nanostructures, and regain the original waveguiding properties of
the nanoribbon, the devices can be treated with a solution of aqua
regia (1:1 HNO.sub.3/HCl:H.sub.2O) (FIG. 25C). This allows the
ribbons to be recycled by stripping them of any residual molecules
or metallic contaminant.
[0203] In addition to simple index measurements the immobilized
metal particles can be utilized as substrates for surface enhanced
Raman spectroscopy, where monochromatic light (532 nm) from the
waveguide or from an external source excites surface plasmons. This
excitation is responsible for large increases in the Raman
cross-section of molecules near or adsorbed to the metal particle,
allowing the collection of vibrational signatures from analytes
that are otherwise undetectable with traditional Raman techniques.
Resonant SERS occurs when both the analyte (in our case a dye
molecule) and the metal plasmons are excited by the same wavelength
of the light. To demonstrate resonate SERS we exposed the
nanoribbons to a 100 .mu.M solution of Rhodamine 6G
(.alpha..sub.max=535 nm) after decorating the waveguide surface
with Ag nanocubes 114. The SERS signal was probed either directly
with a diffraction-limited confocal spot focused on the waveguide
(FIG. 26B--SERS R6G Confocal trace) or via waveguided light (FIG.
26B--SERS R6G WG trace). In the former configuration the waveguide
acts simply as a supporting scaffold for the SERS-active particles.
Here the power at the sample is approximately 2 .mu.W. In the
latter configuration the waveguide channels the excitation to the
particles. Due to the coupling geometry the power accessible to the
particles is less than 100 nW. Performing SERS with large
(.about.500 nm) and small (.about.150 nm) diameter nanoribbons
(FIG. 26B) confirm that a stronger Raman signal is achieved when a
larger number of particles are scattering in the field of view. The
thinner nanoribbon has about 20 nanoparticles contributing to the
signal whereas the thicker ribbon has more than 30 nanoparticles.
The number of particles attached to the surface of the waveguide
was counted from CCD images under dilute nanoparticle
concentrations and slow flow rates (0.75 mL/hr).
[0204] To detect a nonresonant SERS signal from the analyte, we
modified the surface of the metal with 1-dodecanethiol ligands,
which readily assemble into a monolayer on the metal surface. The
SERS spectra in FIG. 26C clearly show the C-C stretching modes of
the alkyl groups at 1080 and 1122 cm.sup.-1 under confocal and
waveguided excitation. The pump powers used were similar to that
described above for the Rhodamine 6G SERS. Interestingly, the
signal-to-noise ratio is nearly identical for both excitation
geometries, despite a lower power density for the waveguided
excitation. However, the diameter of the confocal beam waist and
differing number of particles in each experiment makes a
quantitative comparison between the two configurations difficult.
Future experiments will be needed to conclude if the evanescent
field scattering is a more efficient mechanism for exciting
plasmons in a metallic particle attached to a subwavelength
waveguide. Nevertheless, the ability to extract chemical
information using a subwavelength fiber is a critical step in the
development of compact analytical devices.
[0205] As can be seen, we have demonstrated a novel photonic sensor
based on subwavelength nanowires that is capable of detecting
molecules in solution by absorbance, fluorescence and SERS. The
future of portable all-optical sensors hinges on the provision of
cheap, fast, reliable detectors capable of deconvoluting complex
mixtures. An imperative step in this process is the addition of
chemical specificity to the sensor while simultaneously providing a
multiplexed geometry for high-throughput analysis. Device
portability will certainly benefit from the advent of on-chip
microcavity lasers and the continual efforts of integrating both
active and passive optical elements on a single photonic chip. Use
of the evanescent field to guide light and perform spectroscopy
will undoubtedly play a major role in the design of compact optical
sensors. The initial results shown here are promising for the
development of on-site analytical experimentation, field detection
of biochemical toxins and portable analysis of water
contaminants.
[0206] Although the description above contains many details, these
should not be construed as limiting the scope of the invention but
as merely providing illustrations of some of the presently
preferred embodiments of this invention. Therefore, it will be
appreciated that the scope of the present invention fully
encompasses other embodiments which may become obvious to those
skilled in the art, and that the scope of the present invention is
accordingly to be limited by nothing other than the appended
claims, in which reference to an element in the singular is not
intended to mean "one and only one" unless explicitly so stated,
but rather "one or more." All structural, chemical, and functional
equivalents to the elements of the above-described preferred
embodiment that are known to those of ordinary skill in the art are
expressly incorporated herein by reference and are intended to be
encompassed by the present claims. Moreover, it is not necessary
for a device or method to address each and every problem sought to
be solved by the present invention, for it to be encompassed by the
present claims. Furthermore, no element, component, or method step
in the present disclosure is intended to be dedicated to the public
regardless of whether the element, component, or method step is
explicitly recited in the claims. No claim element herein is to be
construed under the provisions of 35 U.S.C. 112, sixth paragraph,
unless the element is expressly recited using the phrase "means
for."
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